Nelson Mandela's 1990 release marked in South Africa
In Cape Town, prominent figures took part in a commemorative walk at the prison where he spent the final months of his 27-year imprisonment.
Mr Mandela, now a frail 91-year-old, is expected to make a rare public appearance on Thursday evening.
He became the country's first black president in 1994.
Mr Mandela spent most of his sentence in Robben Island prison, off the coast of Cape Town, and later in Pollsmoor Prison on the mainland.
Before his release, he lived in a cottage in the grounds of Victor Verster prison in a rural area some 50km (31 miles) from Cape Town, with his own cook.
Thursday's re-enactment walk went through the gates of Victor Verster prison, now known as Drakenstein prison, where a statue of Mr Mandela stands with its hand upraised.
Hundreds of people, some wearing yellow T-shirts bearing Nelson Mandela's image, retraced his final walk to freedom after 27 years in captivity, chanting "Viva Madiba [his clan name]". Thousands more gathered to watch.
Led by several ANC leaders who spent time in Robben Island prison with Mr Mandela, they marched from the cottage in Victor Verster prison, now known as Drakenstein Prison, where he spent his final months.
In a picture that would have made Mr Mandela proud, black, white and Asian South Africans marched side-by-side through the prison gates, which then closed slowly behind them.
As they passed the statue which has been erected of the prison's most famous former inmate, many raised their fists - recreating the image which has come to symbolise the end of white minority rule in South Africa after years of bitter struggle, led by Mr Mandela.
Cyril Ramaphosa, who was among the veterans of the anti-apartheid struggle taking part in the walk, recalled Mr Mandela's crucial role.
"We are celebrating a life that has been lived in service of our people," he said.
"He knew he needed to continue living for the people that were outside. Without the struggle of our people, Madiba would have never been released," he added, using Mr Mandela's clan name.
Mr Mandela's former wife, Winnnie Mandela, had been due to lead the walk, but a spokesman said on Thursday morning that she would not be appearing because it would have been "too painful".
Poppy Shabalala, a 65-year-old local resident, said she had turned out to celebrate Mr Mandela's legacy.
"He did the unthinkable," she said. "Mandela united black and white people and ended apartheid. I am here today to show my gratitude for what he did."
Mr Mandela did not join the walk, but he is expected to attend a state of the nation address by South African President Jacob Zuma later.
“ If we really want to make a difference we must recapture the spirit of that day ”
Archbishop Desmond Tutu
"We are trying to ensure that he gets a lot of rest during the day so he could be fresh and energetic in the evening to attend parliament," said his grandson, Mandla Mandela.
Mr Mandela was sentenced to life imprisonment in 1964 for plotting to overthrow the government by violence.
During his years in prison he became an international symbol of resistance to apartheid.
In 1990, the South African government responded to internal and international pressure and freed him, at the same time lifting the ban against the anti-apartheid African National Congress (ANC).
Christo Brand, the former prison warden assigned to guard Mr Mandela, said of events 20 years ago: "I hoped there would be no bloodshed. There was no bloodshed. Everything worked out perfectly.
"And I know the way Mandela does negotiations, he was really thinking of the other side, too.
"He not only thinks of the black people of the country, but thinking also of the whites and studying and feeling the fears of the whites in this country.
"And I think through that fear, he came up and thought of a good solution for South Africa."
Archbishop Desmond Tutu, another key player in the fight against apartheid, said the day of Mr Mandela's release was "a day that promised the beginning of the end of indignity".
BBC News Special: Mandela release - 11 February 1990
But he added that while much had been achieved, more remained to be done.
"If we really want to make a difference we must recapture the spirit of that day of Nelson Mandela's release," he said.
In 1991 Mr Mandela became the ANC's leader. He was president of South Africa from 1994 until 1999, when he stood down - one of the few African leaders at the time to voluntarily give up power.
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