On the wall of Thabo Mbeki's study at Highstead, his home on the Groote Schuur estate in Cape Town, is a photograph. A group of young, well-dressed men and women advance in an exhilarating "V" towards the camera, their triumph at having arrived safely in exile captured by the silvery hyperreality of the print, as if black people have found themselves inexplicably in some grand, '50s Hollywood epic. The date is November 1962, the place Dar es Salaam. Thabo Mbeki's group of 28 students has finally landed in the newly independent African state of Tanganyika, the land of uhuru, their springboard to freedom.

They have just been through two months of hell. Masquerading rather inadequately as a football team (they did not even have balls or jerseys), they were stopped by the authorities before even making it to the Bechuanaland border. When they eventually got through Bechuanaland and across to Rhodesia, they were spotted and arrested, and spent six weeks in a Bulawayo jail, waiting to find out if they would be deported back to South Africa. They were eventually dumped back in Bechuanaland, where hostile authorities granted them political asylum in a few square kilometres of open bush. Finally, the ANC managed to procure them the Dakota DC-3 you see in the background of the photograph.

And here they are. Some, like the present Deputy Minister of Justice, Manto Tshabalala-Msimang (front, in black), have their fists in the air. You would never guess, if you did not know, that their leader was that serious young man in the second row, jacket over a T-shirt. He is not participating in the performance of arrival, of resistance, of liberation; he is frozen, rather, in his most habitual pose - that of listening, with a slight, critical scepticism creasing his brow.

And he is looking straight at the camera. Why has Thabo Mbeki kept this image over his desk for the years of his deputy presidency? Perhaps because it reminds him that he was then, as he is now, poised on a cusp; that to save himself and his people, he needs now, as he did then, to keep his eyes on the camera, on the prize, on his destiny.

People will tell you that Thabo Mbeki is without hubris, that he has submerged all personal ambition into the destiny of his movement and his country. People will tell you that he is, and has always been, ruthlessly ambitious, skilful at disposing of all adversaries in his path.

Neither position is particularly useful in understanding the next President. Far more helpful is the knowledge that he loves chess: he has a computerised game he plays to unwind, and a chess set was what he sent his son, Kwanda, from exile, when he heard from his mother, Epainette, Kwanda's guardian, that the boy was a maths whiz.

If Thabo Mbeki's leadership sometimes appears to be without the performative Buwa! ("Tell it!"), the raised fists that characterise the evangelical ANC, it is because he takes his pleasure not in touching the people, the way Nelson Mandela does, or in leading the assault, the way Chris Hani might have, but in outwitting his opponents and realising his objectives as if politics and South Africa's destiny (and, indeed, the field of his own ambition) were one giant chessboard. "Queen to Knight Five," you can imagine him saying to himself, with the quiet satisfaction of an impending checkmate, as he descends the Dakota DC-3 into the heat of Dar es Salaam.

By the time we see Thabo Mbeki in this photograph, all of 20, he is already precious cargo. For most of the 18 months he lived in Johannesburg before departure, he stayed in the home of ANC secretary-general Duma Nokwe. From Dar es Salaam, he will fly with Kenneth Kaunda to London, where he will become Oliver Tambo's protégé. Partly because of his family's credentials, partly because of the seriousness of his intellect, he has already caught the eyes of "the elders". They have given him the tasks of forming a new student body, the African Students' Association, and of leading this, the first group of ANC students to go into exile.

It has already been decided - by both his father and the ANC leaders - that he will study economics in Britain. His A-levels tutor at Sached in Johannesburg, Ann Yates, has arranged him both a benefactor - the liberal peer and former cleric Lord Tim Beaumont - and a place at Sussex University. ("He believed he should have gone to Oxford or Cambridge," she recalls with good humour, "and was rather annoyed I couldn't arrange it!") In all of this he is worlds away from his comrades in the photograph, none of whom has any idea of the next move. All his comrades have is the vague promise that they will get bursaries to study somewhere in the Soviet bloc and that they will return soon to take up their positions at the helm of a liberated South Africa. Given how many of them died, disintegrated or simply drifted away while in exile, there is something quite moving, even tragic, about the hope in their open faces.

Thabo Mbeki talks, light-heartedly, about how his political career began when he was 10, in Queenstown, while he was living in a large old house on Scanlan Street with his uncle, Michael Moerane, a composer and music teacher. Moerane taught Thabo the flute and included him, with his own six children, in the Moerane community band, The African Springtime Orchestra. The Moeranes, with whom he lived for two years, remember him as a quiet but humorous child, who made them laugh by putting his own satirical words to a spiritual they performed, Every Time I See the Spirit.

This was 1952, the time of the Congress Movement's Defiance Campaign, and Thabo and his cousin Kabeli sold Coke bottles to collect the two shillings sixpence needed to join the ANC. They were turned away, however, and told to come back when they were older. In the current biographical writing on Mbeki, this is mythologised as evidence of his political precocity. Mbeki signifies, though, that his interest in Defiance arose more out of youthful curiosity than commitment to the liberation movement when he talks of how, the same year, his uncle thrashed him and Kabeli for attending the town's Van Riebeeck tricentenary celebrations.

It was only in 1956, when he was in the second form at Lovedale College, that Thabo Mbeki was recruited into the ANC Youth League - by an older pupil named Themba Mqota. (Two decades later, in 1975, Mbeki was to be one of the leaders to expel Mqota, along with his "gang of eight" dissidents, from the ANC.)

Mbeki had, in fact, joined the rival Unity Movement youth group, Soya (Society of Young Africans), upon arrival at Lovedale in 1955, but this was more because of regional affiliation than ideological commitment: he was put into Shaw House with all the other Transkeians, and the way they differentiated themselves from the sophisticated city boys of the Eastern Cape who were "Congressites" and who called them moegoes - country bumpkins - was by joining Soya.

The Lovedale that Thabo Mbeki entered was a powerful, history-laden place; it had been founded by Presbyterian missionaries in the 1820s and was, together with Fort Hare University across the Tyume River, the very fulcrum of black intellectual activity and aspiration. It was replete with English public school traditions: old stone buildings quivering beneath a bell tower that marked time rigidly; corporal punishment and prayer; bad food and good teaching; and afternoons on the playing fields - where the young Thabo, determined to shuck his moegoeness, chose to play rugby, an Eastern Cape sport, rather than soccer with the other Transkeians.

'We were the best and the brightest!" says Thabo Mbeki's classmate Jackson Vena, a librarian at Rhodes University. "But there was a contradiction. While we had the same sort of pride in ourselves as our white contemporaries at St Andrews or Kingswood - we wrote the same exams and got the same results -- we knew we would never have the same opportunities as them. It was that 'you may not' that motivated us. That's where our activism came from."

Certainly, when lights went out in the dormitories, the Lovedale boys did what all boarding-school boys do: joshed each other and fantasised about the girls over at St Matthews. But they also pulled out their copies of New Age and had fierce, whispered debates about the merits of the Congress versus the Unity Movement. On weekends, the youth leaguers stole surreptitiously across the Tyume River to meet under "The Big Tree" at Fort Hare with older activists; here they collected literature which was then distributed - by their exacting branch secretary, Thabo Mbeki - according to a strict roster.

At the centre of Lovedale's life was a paragon of crusty old liberalism, a man who symbolised the best - and worst - of Presbyterian paternalism and who became synonymous with Lovedale, J P "Jack" Benyon. A look at the records of Lovedale at the time Mbeki was there gives a sense of the official attitude to student politics. In 1957, for example, Benyon refused to recommend his head prefect and top scholar, Sipho Makana, to Fort Hare because "I have confidential information that [he has] been tied up with politics while at school". Makana was to leave South Africa in Mbeki's group in 1962 and is now South Africa's ambassador to Russia.

In 1958 Benyon "hesitates" to recommend two of the brightest boys for bursaries because "they have been dabbling in politics". On the report cards of the same year, the work of one Martin Hani (nom de guerre: Chris) is recorded to have "deteriorated", although his character remains "fair"; one Winston (Njongonkulu) Ndungane is dismissed as "lazy and uninterested"!

There are no reports for Thabo Mbeki, because only matric records were kept, and he was expelled in March of his matric year - a consequence of a class boycott that was to sound the death knell of the mission school system in South Africa. Mbeki was one of the leaders of this strike, but though he quickly became a student leader at the college, he was not noticed as such by the teachers.

Sipo "Mac" Makalima, now in his 80s, remembers Martin Hani and Sam Nolutshungu (the late Wits vice-chancellor-designate) vividly as student leaders and hotheads. Thabo, on the other hand, was "one of the brighter young fellows, usually first in the class. But he gave no impression of being a leader. One didn't notice anything in particular about him. He gave no impression at all except that of being a diligent, quiet boy."

Lovedale was on its own cusp. Bantu Education had been instituted in 1954, and although the missionaries still ran the school while Mbeki was there, it was now under the nominal control of the state. Perhaps Mbeki is romanticising matters a bit when he recalls that at the Lovedale he entered in 1955 "you didn't get any sense of racism"; this, perhaps, is the recollection of an inquisitive boy from the countryside, awed by the possibilities and sophistications that this grand institution represented.

By 1959, though, Bantu Education officials had been placed at the school, and Mbeki remembers that the children of one of them would taunt the pupils by calling them "Kaffir" as they went down to the playing fields.

The mission school system that had educated - and thus empowered - his parents and the entire black elite to which he belonged was being dismantled before his eyes.

And so, by the beginning of Mbeki's matric year, tensions between pupils and the authorities were high; both, it seems, were playing out the larger dynamics of society through the usual boarding-school conflicts over bad food and unfair masters.

When Benyon expelled a senior boy - allegedly for stirring the juniors up into insurrection, but actually because the boy, one of his "spies" in the Youth League, had been exposed as a "double agent" - the school erupted, and, in his first real flexing of political muscle, Thabo Mbeki harnessed the energy into a class boycott that was, with a little help from old-fashioned schoolboy intimidation, nearly 100 percent successful.

Almost the whole student body of 800 barricaded itself in the gymnasium and sent a lengthy petition to the school authorities. Eventually, after a few days, the pupils were called into the Great Hall and told to go home; 600-odd did not return. Mac Makalima confirms that the authorities had no inkling of Mbeki's involvement. Mbeki was, in fact, one of the few pupils not expelled outright: he was given the option of returning to the school provided he signed a letter pledging to stay out of politics (he did not oblige).

Thus we have an early and powerful signal of the way Thabo Mbeki works: without leaving fingerprints; without disrupting for a moment his public image of diligence and good behaviour.

In 1946 a violent student uprising at Lovedale came close to destroying the school, the pupils marching through the campus singing Onward Christian Soldiers. In 1976 schools burnt all over the country, pupils marching through townships singing Shaya maBunu (Kill the Boers).

In 1959, though, a well-dressed, well-spoken young man led a quiet, rather dignified class boycott that closed South Africa's premier black educational institution down. Benyon resigned a year later, and the shuffling grey shoes of Bantu Education finally took over.

Thabo Mbeki might have spent his life as a revolutionary, but he has never been a rebel. The generation of activists before him bucked against the stuffy bourgeois conservatism of the ANC in the '40s and later dispensed with non-violence altogether. The generation of activists to follow him, the Soweto Generation, rebelled against the silent collaboration of their parents. With a few hot-headed exceptions, Thabo Mbeki and his peers were caught somewhere between these explosive generations.

Certainly, there are ways that Thabo Mbeki tried to differentiate himself from his legendary father - "I'm not Govan Mbeki," he sharply told other activists when he arrived in Johannesburg in 1961, after having completed his matric studies by correspondence at home in Idutywa - but how do you rebel against Govan Mbeki or Nelson Mandela? When your parents themselves are taking up arms, what can "militant youth" possibly mean?

Mbeki's first piece of political polemic was published in New Age in January 1962. Here the author talks, in a rather old-fashioned way, about how "the African student" must take the lead and act responsibly as "the intellectual elite of a people [suffering] from subjection by a minority government". The article was a report of the launch of the African Students' Association in Durban. There is a gangly adolescence to its prose - "The student has begun to feel that he must grapple with himself, and, without self-pity, throw his might against his disabilities" - but in its articulation of the need for a vanguard of African students, it is a prescient foreshadowing of the Black Consciousness Movement that was to follow a decade later.

During that trip to Durban Mbeki met Ronnie Kasrils, who was struck by "this extremely pleasant, well-dressed, well-mannered, self-confident young man". Kasrils invited him to a party at his cottage: "We made one helluva noise, singing and dancing. It was a really hot party, racially mixed, the police eventually came and raided it. But Thabo wasn't exuberant and exhibitionist on the dance floor the way the rest of us were. He was more of a guy who talked to people."

In Johannesburg, Mbeki had connected with the young Congress Movement activists at Macosa House, just down the road from Tambo and Mandela's law office. The ANC, already banned, met surreptitiously on the top floor of the building, which also served as a recreation centre. One of its denizens was the young Essop Pahad, then a student at Wits, who was to become Mbeki's closest friend: "My early recollection of him," says Pahad, "is of an exceptionally charming fellow, good-looking - lots of women used to run after him, of all races and colours - and someone who was clever and confident."

Clearly, the young Thabo had exfoliated the moegoe entirely by the time he hit Johannesburg, aged 18; in this sophisticated, cosmopolitan environment, he seems to have come into his own. The scene he became involved in, with people like Pahad, was progressive and non-racial; even as leaders were being detained (Thabo's own father was in and out of jail) and Nelson Mandela was underground organising armed resistance, life in Johannesburg included going off to white girls' parties in the northern suburbs, drinking, dancing and talking about revolution.

Kasrils remembers that Mbeki's favourite accessory - his only nod to flamboyance - was a Lenin pin that he wore on his lapel. Just as Govan and Epainette Mbeki were attracted to the Brave New World of communism when they went to Durban in the '30s, Thabo Mbeki would inevitably be drawn to it in Johannesburg in the early '60s. He was recruited to the SA Communist Party by his mentor, Duma Nokwe, in 1962, and he read Marx in a study group under the tutelage of Bram Fischer, whom he remembers as "a lovely old fellow, a very good man, a good teacher".

As we shall see, Mbeki later moves away from communism and eventually leaves the party.

As a young man, though, he explains, "you would naturally, without thinking, try and get some understanding of South Africa. In the Youth League you'd discuss [things like] the history of the evolution of apartheid, the colonial legacy, etc. But you were in a process of seeking greater knowledge. Being a member of the communist party with its Marxist-Leninist philosophy takes you a step further to say, 'Let's study societies!' Not just apartheid society, but the evolution of all societies."

And so, in Johannesburg, under the tutelage of people like Fischer, Nokwe and senior ideologue Michael Harmel, Mbeki read about how China was uplifting its peasantry; about how the Soviet Union had stamped out illiteracy. "Whereas in the ANC you were fighting for a liberation of the oppressed masses, you were doing that in order to achieve the emancipation of the peasants, of the working people. So that would necessarily lead into the study of society." In other words, into Marxism.

The communist party opened up new worlds, wider horizons, for the 20-year-old Thabo Mbeki. It put him into the vanguard - the very elite - of the liberation movement, but it also put him into a thrilling new world of ideas. And so, when he stepped off that Dakota DC-3 in Dar es Salaam, he was not only someone who was walking to the camera, to the future, with an unshakeable sense of destiny for himself and his people; he was also a young man for whom a door had been opened, for whom a lifetime of intellectual excitement awaited; someone who was, as Ann Yates puts it, "awake; brave with ideas, not afraid of new thoughts".

We have already seen how Thabo Mbeki articulates the tension between the ANC's quest for modernity on the one hand and its pull to atavistic "family values" on the other. As he steps off that plane in November 1962 (as he steps up to the podium to take the oath of presidency next month) he embodies yet another paradox.

On the one hand, there is the certitude of history, of struggle, of a chess game that will either be won or lost. On the other, there are whole libraries of possibility, infinite bookshelves from which any idea may be assimilated. Waiting for him in England there is Brecht and Beethoven; there is Shakespeare, there is Yeats.

With acknowledgements to Mark Gevisser and the Sunday Times.