The ceremony at London's City Hall for Holocaust Memorial Day.
The ceremony included a talk by Amonon Weinsteain, who restores violins which belonged to Jews in the ghettos and were played at the concentration camps. Violinists Charlie Siem and Victoria and Jack Liebeck then performed on the violins.
The Mayor of London gave a reading of Elie Wiesel's Never Shall I Forget and the crowd of dignitaries also heard testimony from two Holocaust survivors, Gena Turgel MBE and Zigi Shipper.
Mr Johnson, Mrs Turgel and Mr Shipper signed the Book of Commitment and lit a memorial candle.