Before an audience of politicians, writers and dignitaries, he twice paused during his address. As he voiced the apology, the US civil rights leader the Rev Jesse Jackson walked over and placed his arm around the mayor. Mr Livingstone completed the long awaited statement, dabbing tears from his eyes, his voice shaky.
Before leaving office Tony Blair expressed "regret" for Britain's involvement in the slave trade, but he was criticised by some - including Mr Livingstone - for not going further. In March the mayor called on the institutions to say sorry.
Yesterday, to wide applause at City Hall, he said: "As mayor I offer an apology on behalf of London and its institutions for their role in the transatlantic slave trade. Some say that recognising such a crime is a form of - and I quote - 'national self hate'. But the late Senator Bobby Kennedy often quoted the French writer Albert Camus who wrote: 'I should like to be able to love my country and still love justice'."
He said London continued to benefit massively from the fortunes made by slavers. He added: "It was the racial murder of not just those who were transported but generations of enslaved African men, women and children. To justify this murder and torture black people had to be declared inferior or not human. We live with the consequences today."
Afterwards he said he had read the address several times before delivering it but "had no idea how overwhelming it was going to be".
Apologies have been made by the city of Liverpool and the Church of England synod. But earlier this year the Conservative leader, David Cameron, appeared to dismiss the idea, saying: "I don't actually think that one generation can meaningfully apologise for something that a previous generation did."
The mayor, who was speaking at London's first Annual Slavery Memorial Day ceremony, called on national leaders to accelerate their plans to have a UK wide event each year. The date marked yesterday coincides with the great slave rebellion in Haiti on August 23 1791, and is the date preferred by Unesco.
Francoise Riviere, assistant director general for culture at Unesco, heaped praise on the mayor. "You have distinguished yourself as the first high-visibility elected official to take such a historic stand, thereby setting an example."
Aidan McQuade, director of Anti-Slavery International, said: "This will help increase pressure for a formal apology from the government and an annual national slavery memorial day."
