London's famous landmarks were on Sunday night lit up in the colours of the French national flag in tribute to the 17 people killed in the Paris terror attacks.
The Tower Bridge, Trafalgar Square and National Gallery in central London were streamed with the blue, white and red of the French tricolour.
It coincided with British Prime Minister David Cameron's visit to Paris to join other world leaders for a mega unity march.
"We're here to demonstrate that we all stand for the values of democracy, freedom, freedom of expression and tolerance," he said.
"We in Britain face a very similar threat -- a threat of fanatical extremism -- and we have to confront that in every way we can," he added.
Cameron was among dozens of world leaders who linked arms at the head of the march, along with the victims' families.
UK Home Secretary Theresa May also joined EU interior ministers at a meeting to discuss the response to the attacks.
Across Britain, hundreds gathered at Trafalgar Square in London, Millennium Square in Leeds and Liverpool's Derby Square to show solidarity with the French people.
The Trafalgar Square fountains rotated the colours of the French flag and another major landmark, the London Eye, went dark with the French colours projected on to County Hall behind it.
London mayor Boris Johnson said Londoners had been "appalled by the distressing scenes in France" and it was important to pay tribute.
Eleven people were killed by Al-Qaeda terrorists at the offices of the satirical magazine 'Charlie Hebdo' on Wednesday, with a policeman shot dead nearby. Four hostages died at a Jewish supermarket on Friday.
Cameron earlier tweeted to say he was on his way to Paris to "march with the French people".
"The #CharlieHebdo murders will not crush our spirit or our values," he said.
Labour leader Ed Miliband, who is also going to Paris, said, "All of us have been shocked and appalled by the events in France, by the murderous terrorism that this represents.
"We've been inspired by the response of the French people, and indeed by the response of people across faiths, across communities, in this country.
French Ambassador to the UK Sylvie Bermann said she was grateful for the UK's show of solidarity.
"A lot of people also came to the embassy and signed our condolence book and also sent flowers -- like the mayor of London -- and I understand that Nick Clegg will participate in our rally in Trafalgar Square," she said.
