Tower Bridge will be illuminated with the colours of the French tricolor as London stands in solidarity with its sister European capital.
Vigils are being held in Trafalgar Square as people in the city come together to mourn those killed and injured in the Paris attacks.
Organisers of the Lord Mayor’s Show have cancelled Saturday’s fireworks display over the River Thames as a mark of respect and are asking people not to travel for them.
Flags in London, including at Downing Street, Guildhall and the Old Bailey, are flying at half-mast, and flowers and candles have been laid on the steps of the French Embassy.
Mayor of London Boris Johnson has also written to his counterpart in Paris, Anne Hidalgo, to convey the sense of “shock and grief of millions of Londoners at the tragic events”.
More than 7,000 people were expected to turn out for the Lord Mayor’s Show on Saturday, with a procession of 70 floats through London to mark 800 years of the annual event.
Jeffrey Mountevans, the Lord Mayor, led a two-minute silence at the Mansion House at 11am at the start of the procession.
The City of London Corporation said: “As a mark of respect to the Paris victims we will light Tower Bridge in colours of the Tricolor instead of annual #LMS800 fireworks.
“It is time for a show of solidarity with the victims of an atrocious terrorist attack and not a time for celebrations.”
Vigils were being held at Trafalgar Square at 2pm and 9pm, echoing those held after the Charlie Hebdo terrorist attacks in Paris in January.
Between 300,000 and 400,000 French people are thought to live in London, making it France’s sixth biggest city by population.
Mr Johnson said: “We regard Paris as our sister city, a place for which we have deep reserves of love and admiration and respect. We are two capitals united in our values - democratic freedom, openness and tolerance.
“The people who launched the attacks last night have no such values. They wish to undermine the things we hold most precious. They want to set neighbour against neighbour. They want to spread fear.
“They will not succeed. As President Hollande has rightly said, they must and will be defeated. We stand ready in London to do everything in our power - to do whatever it takes - to assist in a struggle that embraces us all.”
Faith communities have called for a third vigil at the same landmark for those killed and injured in Paris.
Organised by the Christian-Muslim Forum and the Muslim Council of Britain (MCB), people are asked to bring tea lights and red, white and blue flowers to Trafalgar Square, which will fall silent at 6.30pm.
Catriona Robertson, director of the Christian-Muslim Forum, said: “Terrorism has no religion - and today’s event will show solidarity and how we are all united in our prayers for those killed and injured.”
MCB secretary general Dr Shuja Shafi added: “The attacks once again in Paris are horrific and abhorrent, and we condemn this violence in the strongest possible terms.
“This attack is being claimed by the group calling themselves ‘Islamic State’. There is nothing Islamic about such people and their actions are evil, and outside the boundaries set by our faith.”
The fountains in Trafalgar Square will also be lit red, white and blue tonight, and two French Tricolores will be projected onto the front of the National Gallery.
Buildings around the world have been lit with the French colours, including the Sydney Opera House, One World Trade Center in New York and the Sky Tower in Auckland, New Zealand.