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.