Mourners stood in Trafalgar Square in front of The National Gallery – which had projections of the French flag across its facade – at 9pm.
The crowd held their mobile phones aloft, the light of their screens illuminating the darkness, during a minute's silence.
Some people lit candles and laid flowers to and others held the French tricolor – 24 hours on from the attack.
Even the fountains in the world-famous square were lit up with blue, white and red lights as darkness fell.
Many of those at the vigil were French people living in Britain and led emotional sing-alongs of the national anthem.
The capital is home to an estimated 300,000 French nationals, according to recent statistics.
Many flags in London are flying at half-mast in tribute to the victims.
And flowers and candles have been laid on the steps of the French Embassy near Hyde Park.
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