Concerns remain over a “handful” of Britons caught up in the Paris attacks, Home Secretary Theresa May has said.
One Briton – Nick Alexander from Essex – has been confirmed dead, and Mrs May said others had been injured.
She warned people in Britain to be “alert but not alarmed” over security and said increased police patrols and border searches were taking place.
Mrs May later chaired a Cobra emergency committee meeting about the lessons to be learned from Friday’s attack.
She told the BBC’s Andrew Marr Show the UK stood “in solidarity” with France, adding: “The terrorists will not win – we will defeat them.”
Asked if there was a security threat from Islamic State (IS) militants hiding among migrants coming to the UK, Mrs May said Britain only took the most vulnerable people from Syrian refugee camps, and rigorous checks were in place.
She later said that a “crisis team” had been sent from the Foreign Office to Paris to assist British citizens who had been injured and there would be support for those returning who “have been part of witnessing these terrible events”.
The British ambassador to France, Sir Peter Ricketts, told the BBC it was hard to be exact about the number of injured Britons as people were in various hospitals across Paris.
IS militants have claimed responsibility for the attacks.
Mr Alexander, 36, from Colchester, died in the attack at the Bataclan concert hall, where he is thought to have been selling merchandise.
His family described him as “generous, funny and fiercely loyal”.
Friends of Mr Alexander also paid tribute to him on social media. Joe Trohman, lead guitarist of the rock band Fall Out Boy described him as a “great guy”.
The Bishop of Chelmsford, the Right Reverend Stephen Cottrell, said he was praying for Mr Alexander’s family and had sent a message of support to their local church.
A UK government source said an unspecified number of Britons were being treated in hospitals in France.
More than 80 people are believed to have died at the Bataclan concert hall, one of the locations targeted as gunmen and suicide bombers carried out attacks.
The US band Eagles of Death Metal were playing a gig when attackers burst into the venue and opened fire, but the band themselves survived unscathed.
Other bands, including U2, Motorhead and Foo Fighters, have cancelled gigs in Paris in the wake of the attacks.
By Susana Mendonça, BBC political correspondent
The terror attacks in France may not have altered the threat level in Britain yet, but what’s clear from the home secretary’s remarks is that it’s a fluid situation which could change as the French investigation unfolds.
The threat level has been set at “severe” in Britain for more than a year now, but since the shocking events in Paris she said there had been an increased police presence on Britain’s streets and tighter border checks – with more freight vehicles being screened.
But the overriding message coming from Mrs May was one of reassurance and of solidarity.
She told Brits to be “alert but not alarmed” and – with a handful of possible British victims in the Paris attack – she reiterated that Britain would stand shoulder to should with France.
Michael O’Connor, from South Shields, was at the Bataclan concert hall when gunmen attacked.
The 30-year-old said he lay on top of his girlfriend to protect her as other audience members were shot and killed. Both escaped unharmed after playing dead.
“My girlfriend was so brave though, she kept on telling me this isn’t where it’s going to end, we’re not going to die here,” he said.
In other developments:
- Landmarks across the UK, including London’s Tower Bridge and Edinburgh Castle, were lit up in the colours of the Tricolour
- Flags were flown at half mast at Downing Street and the French Embassy in London
- A vigil has taken place in Trafalgar Square. Groups taking part included the Muslim Council of Britain and the Christian-Muslim Forum
- Prayers are being said at religious events across Scotland, with a memorial service planned at St Giles’ Cathedral in Edinburgh
- The London School of Economics tweeted that one of its former students, Valentin Ribet, had been killed. He was a French national
Talks about the security threat are expected later after leaders, including David Cameron, from the G20 countries meet in Turkey.