Boris Johnson paid tribute to the bravery of Holocaust and Rwandan genocide survivors who addressed the City Hall Holocaust Memorial Day event on Tuesday.
The speakers were Mala Tribich, who was in Ravensbruck and Bergen-Belsen, and Alphonsine Kabagabo, who survived the Rwandan genocide in 1994.
Polish-born Mrs Tribich recalled Bergen-Belsen as "something out of hell. People would just collapse and die as you were speaking to them - there were huge piles of twisted, decaying corpses." She recounts her experiences to school groups because "it is vital that young people learn about what can happen when tolerance is lost".
Mr Johnson said Mrs Tribich "reminds us that the Holocaust happened because other people let it happen. People from across the world have chosen to make the capital their home because of the tolerance, diversity and welcome that is synonymous with this city.
"Sadly, like my great grand-father who came to England during his exile from Turkey, many people have not made London their home through choice, but as a refuge from the persecution they face elsewhere. We must learn from their struggles and stand firm in challenging oppression wherever and whenever it occurs."
Rabbi Jeremy Gordon of New London Synagogue said kaddish and Mrs Tribich and Ms Kabagabo lit a memorial candle.
Afterwards, Holocaust Educational Trust chief executive Karen Pollock expressed disappointment that London Assembly and ex-BNP member Richard Barnbrook had chosen to attend the event.
"A vital lesson of the Holocaust is the value of freedom, democracy and human rights," she said.
As an Assembly member, Mr Barnbrook was entitled to be present. But it begged the question "of why someone who has spent their political life consorting with racists and Holocaust deniers would want to attend a Holocaust commemoration"?
Rejecting the claim, Mr Barnbrook responded: "My mother is half-Jewish, and my grandmother fled Germany in the 1930s. I came because genocide happening anywhere is abhorrent, it's disgusting. And if people don't like me there because of my politics then all I can say is tough. I have been asked about [BNP leader] Nick Griffin's questioning of the numbers who died in the Holocaust before. I told him I thought he was wrong and he said he regretted saying it."
Mr Barnbrook, was one of the BNP's senior figures before his expulsion following the May 2010 election after calling for an investigation into alleged corruption among party officials.