On 11 February 1990, the then president of South Africa, FW de Klerk, took the fateful decision to release Nelson Mandela, the charismatic hero of the struggle against apartheid. Twenty years on, he talks about the circumstances that led him to set the world's most famous political prisoner free, launching a new era in a divided country

After 26 years in captivity, Nelson Mandela did not want to be set free straight away. Two days before his release, the world's most famous political prisoner was taken to see President FW de Klerk in his Cape Town office. The president got a surprise.

"I told him he would be flown to Johannesburg and released there on 11 February 1990. Mr Mandela's reaction was not at all as I had expected," said De Klerk. "He said: 'No, it is too soon, we need more time for preparation.' That is when I realised that long hours of negotiation lay ahead with this man."

Twenty years after the event, sitting in the study of his Cape Town home, Frederik Willem de Klerk, now 73, still has the headmasterly style and deliberate speech that the watching world came to know as he played a crucial role in dismantling apartheid. But the winner of the 1993 Nobel peace prize still recalls the enormous leap of faith that was required to negotiate the end of white minority rule with what he describes as the "fundamentally socialistic" African National Congress of the time.

Just after 4pm on the date appointed by De Klerk, Mandela, then 71, walked free, holding the hand of his wife, Winnie. The prisoner had lost his argument for a later release date but had persuaded De Klerk to allow him to leave directly from Victor Verster prison, in Paarl, near Cape Town. Mandela held up his fist in an ANC salute. In an instant he switched from being a symbol of the oppressed to the global symbol of courage and freedom that he remains today.

Mandela's release did not signal the end of apartheid. In fact, the white-ruled pariah state was entering the most dangerous chapter in its history since the introduction of racial separateness in 1948.

Four hours after leaving prison, Mandela arrived in Cape Town to address thousands of people gathered outside city hall. The impatient crowd had clashed with police and bullets had been fired. But Mandela did not bring a message of appeasement. "The factors which necessitated armed struggle still exist today," he told the cheering onlookers.

Mandela called on the international community to maintain its sanctions. "I have carried the idea of a democratic and free society in which all persons live together in harmony and with equal opportunities. I hope to live to see the achievement of that ideal. But if need be, it is an ideal for which I am prepared to die," he shouted.

With hindsight, Mandela used the fiery address to take up a negotiating position and convince the black majority that he had not made a secret pact with the authorities.

De Klerk had his moment of truth nine days earlier, in an address to the all-white parliament that coined the phrase "a new South Africa". "There were gasps in the house, yes," said De Klerk, "but not at the news of Mr Mandela's release. The gasps came when I announced the unbanning not only of the ANC but also the South African Communist party and of all affiliated organisations, which included the armed wing of the ANC, Umkhonto we Sizwe. There were gasps then and, from the far-right party, protests and boos."

De Klerk speaks slowly and clearly – and charmlessly. He is a lawyer from a strict, Calvinist tradition in which displays of emotion are a seen as a sign of weakness. His one quirk seems to be the incessant chewing of gum. He has lived in this modern house in Fresnaye for 18 months, having moved into Cape Town with his second wife, Elita, from his farm in Paarl. He points out that, from his garden, he has a view of Robben Island, where Mandela spent 18 years in prison. It is a fact. He does not reveal whether it leaves him hot or cold.

But radical change requires steely nerves. De Klerk had become president in September 1989, the son of a National party cabinet minister and the nephew of a prime minister. He grew up with Afrikaner fear in his DNA – the dread that after 400 years on the tip of Africa and the struggle against British colonial rule, his Huguenot descendants would be chased into the sea by the black majority. That fear contributed to policies that built his nation – forced removals to create racially segregated areas and blacks being deprived of their citizenship. It led to "passbooks", introduced to restrict black people's movements beyond those that were necessary to the economy, and separate beaches, buses, hospitals, schools, universities and lavatories for blacks, whites, mixed-race "coloureds" and Indians.

As he prepared his 2 February speech at his holiday home in Hermanus in the Western Cape, De Klerk claims he had no confidant. "My predecessor, PW Botha, had an inner circle and I did not like it. I preferred decisions to evolve out of cabinet discussions. That way we achieved real co-ownership of our policies."

He says his consultative style was a break with National party culture. But he also claims – in a line of argument that allows him to avoid condemning apartheid outright – that the system unravelled through a gradual process. Even today, he admits only that international sanctions against South Africa "from time to time kept us on our toes".

In 1959 prime minister Hendrik Verwoerd's government divided black South Africans into eight ethnic groups and allocated them "homelands" – nations within the nation. The move was a cornerstone of an Afrikaner nationalist dream to create a republic, but it led to international isolation. De Klerk was a vigorous supporter. "I wanted us to take a more adventurous approach to the nation state concept, but the project ultimately failed because the whites wanted to keep too much land for themselves.

"The third phase – which coincided with my entering cabinet but was not started by me – was a shift towards reform. It focused on making separate development more acceptable while still believing it was just. But by the early 1980s we had ended up in a dead-end street in which a minority would continue to hold the reins of power and blacks, outside the homelands, really did not have any meaningful political rights. We had become too economically inter-dependent. We had become an omelette that you could not unscramble."

In 1986 the National party abandoned the concept of separate development. "We embraced the idea of a united South Africa with equal political rights for all, but with very effective protection of minorities. Then my predecessor lost his enthusiasm. When I took over, my task was to flesh out what was already a fairly clear vision, but we needed broad support. We needed negotiation."

De Klerk moved quickly. In October 1989, a month after succeeding Botha, he released Mandela's political mentor, Walter Sisulu, and seven other prominent Robben Island prisoners. De Klerk says: "When I first met Mandela we did not discuss anything of substance, we just felt each other out. He spent a long time expressing his admiration for the Boer generals and how ingenious they were during the Anglo-Boer war. We did not discuss the fundamental problems or our political philosophies at all.

"Later, during the negotiations, it became clear that there was a big divide. On the economic side, the ANC was fundamentally socialistic, the influence of the Communist party was pervasive and they wanted nationalisation. They also wanted to create an unelected government of national unity which would organise elections. We insisted on governing until a new constitution had been negotiated and adopted by parliament."

De Klerk's successive negotiated victories potentially saved South Africa from the post-colonial governance void suffered by many other countries on the continent. They also entrenched minority rights constitutionally and set the country on a capitalist path. "The government that came into power after the April 1994 elections was going to need a budget. It was drafted by our finance minister, Derek Keys, and he convinced them of the necessity to stay within the free-market principles that had been in force in South Africa for decades. The ANC has stuck to these principles and that is one of the great positives."

He worries that the left wing of the governing alliance – which supported President Jabob Zuma's offensive to oust Thabo Mbeki in 2008 – will win its current campaign for payback. De Klerk, who retired as deputy president in 1997, also believes South Africa is ripe for a political shake-up, maybe as soon as in next year's municipal elections.

"You cannot say we are a healthy, dynamic democracy when one party wins almost two-thirds of the vote. We need a realignment in politics. I am convinced there will be further splits in the ANC because you cannot keep together people who believe in hardline socialism and others who have become convinced of free-market principles. The 2011 elections will be the opportunity for some much-needed shock therapy. I hope people at those elections will use their right to vote less with emotion and more through reason to express their concerns about the failure of service delivery."

The foundation he runs in Cape Town officially exists to defend the constitution but places a strong focus on minority rights – those of Afrikaners and the Afrikaans-speaking "coloured" population. "The ANC has regressed into dividing South Africa again along the basis of race and class. We see an attitude in which for certain purposes all people of colour are black, but for other purposes black Africans have a more valid case in the field of, for example, affirmative action than do brown or Indian South Africans. The legacy of Mandela – reconciliation – urgently needs to be revived."

He says some whites still accuse him of having given the country away. Asked what would have happened had he not made the 2 February speech, De Klerk has a ready answer. "To those people I say it is a false comparison to look at what was good in the old South Africa against what is bad today.

"If we had not changed in the manner we did, South Africa would be completely isolated. The majority of people in the world would be intent on overthrowing the government. Our economy would be non-existent – we would not be exporting a single case of wine and South African planes would not be allowed to land anywhere. Internally, we would have the equivalent of civil war."