Anti-apartheid campaigners paid tribute to the man they spent decades trying to free at a Nelson Mandela vigil in central London.
Former members of the Anti-Apartheid Movement (AAM) spoke of their grief at his death and their relief that he was at peace after a long illness.
They and several hundred supporters gathered outside South Africa House in Trafalgar Square, which once played host to demonstrations against apartheid.
Suresh Kamath, the AAM vice chairman who organised momentous Mandela tribute concerts at Wembley in 1988 and 1990, said the building had once symbolised "everything we fought against".
He said: "He was an iconic figure to me, someone I looked up to when I was a teenager - someone who said to the world that we can overcome the most appalling oppression if we are prepared to work together.
"He was someone who overcame great adversity for the service of his people, and who said to us that people of different nations, of different backgrounds, can work together."
Lela Kagbara, secretary of the Southwark AAM from 1986-94, said: "He was a symbol of the struggle, a focus for anger in a way. We didn't necessarily think that we would win in getting him out, but it was just something we had to do.
"While I was devastated, I was relieved for his sake, because he was suffering, and I'm afraid for what's going to happen next.
"There's been a lot of progress in South Africa but there's a lot left to do, and I'm just hoping that people have the faith that it is possible through peaceful means to get somewhere."
Crowds chanted Viva Madiba and sang Nkosi Sikelel' iAfrika, the anthem of the African National Congress which became South Africa's national song in 1994.
Candles and flowers had been laid at the entrance to the building under a South African flag and a sign reading: "Thank you Nelson Mandela."
Chitra Karve, chairwoman of ACTSA and an AAM activist since 1990, met Mr Mandela when she helped organise his visit to the family of Stephen Lawrence shortly after his murder in 1993.
She said: "His agenda was fully packed, but as soon as he heard what it was about, he made space for a very early morning meeting.
"He came into this quite small room, with the Lawrences all there, obviously still grieving very heavily. What I saw was a very compassionate, caring, very gentle man, who was very respectful of them and their grief."
Jerry Dammers, the founder of The Specials and writer of Free Nelson Mandela, urged people to honour Mr Mandela's legacy by doing the work he started.
He said: "'The best way we can remember him and politicians can remember him - the best tribute they could pay to him - would be to listen to what he said and act on what he said."
He cited a speech at Trafalgar Square in 2005 where Mr Mandela called on the world to abolish "the prison of poverty" by ending African debt and establishing fair trade.
But activists also hit out at those who initially opposed Mr Mandela's release from prison yet have today praised his memory.
Peter Hain, the Labour MP and anti-apartheid activist who survived an attempt on his life by letter bomb in 1972, said there was "a lot of amnesia around Britain's collaboration with apartheid" and blasted the "indifference and complicity" of politicians on the right.
He said Tory chairman Lord Tebbit was "still in absolute denial" for claiming his government had been right to shun sanctions against South Africa.
Lord Tebbit told BBC Radio 4's World At One that the Thatcher government had been "proved right" on South Africa and that Mr Mandela was "the leader of a political movement which had begun to resort to terrorism".
Mr Hain said: "(Mr Mandela) was an ogre, a terrorist ogre, in the eyes of right-wingers and those cosying up to apartheid. But he was always the democratic freedom fighter that we now revere.
"There's a lot of amnesia around Britain's collaboration with apartheid and the failure of successive British governments, mainly Conservative, to really tackle its evils.
"They continued to trade with it and maintain close relations with its rulers - whether on the sports fields, in business, or in culture - while there was a bitter struggle going on by anti-apartheid forces inspired by Nelson Mandela.
"If Mandela could forgive but not forget, I think we can do the same. I certainly don't forget."
In 2006 David Cameron flew to South Africa to tell Mr Mandela his party had made "mistakes" about the ANC and that Baroness Thatcher was wrong to brand them "terrorists".
Mr Kamath said: "We know that the Conservative government in the 1980s gave succour to the apartheid regime, and many Tory MPs were actively supporting the South African government.
"In a lot of ways I think they're a bunch of hypocrites, but also I'm glad that they've seen the light - that they recognise black and white people can live in peace together.
"I really hope that they are sincere and that they are not just saying it because they are politicians."
Tony Dykes, an AAM veteran and the director of successor group Action for South Africa, said: "When they've acknowledged that it was wrong in the 1980s to label the ANC a terrorist organisation, that it was wrong for some members of the Conservative Party to call for Nelson Mandela to be hanged, we should recognise and celebrate that.
"Those who just pretend, well, they know they're pretending and we know they're pretending."
Christabel Gurney, editor of the AAM's official newspaper during the 1970s, said Mr Mandela's "spirit of forgiveness and tolerance" meant activists should be glad he now receives such praise.