Image copyright Alexander family Image caption Mr Alexander died doing the job he loved, his family said

One Briton was among the 129 people killed in Friday's attacks in Paris, the Foreign Office has confirmed.

He has been named as Nick Alexander, who 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".

A government source said there were fears a "handful" of other British people had been killed. An unspecified number are being treated in hospitals.

'Peace and light'

A family statement said: "It is with huge sorrow that we can confirm that our beloved Nick lost his life at the Bataclan.

"Nick died doing the job he loved and we take great comfort in knowing how much he was cherished by his friends around the world.

"Thank you for your thoughts and respect for our family at this difficult time. Peace and light."

Media playback is unsupported on your device Media caption David Cameron said to the French people: ''Your pain is our pain''

The Bataclan concert hall suffered the deadliest attack during the wave of violence in Paris, which involved gunmen and suicide bombers. More than 80 people are believed to have died at the concert hall.

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.

Earlier, Prime Minister David Cameron said Britain's terror threat level was currently "severe", but would be reviewed in light of the attacks.

But he added that "however much we prepare, we in the UK face the same threat" as France.

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Image copyright Lee Patterson Image caption A bridge in Berwick-upon-Tweed is lit up the colours of the French flag

So-called Islamic State (IS) has said it carried out the attacks, and Mr Cameron said it showed the threat from the group was "evolving", with a "new degree of planning and co-ordination and a greater ambition for mass-casualty attacks".

He added: "The terrorists' aim is clear: it is to divide us and to destroy our way of life.

"So, more than ever, we should come together and stand united and carry on with the way of life that we love."

Image copyright Mario Cacciottolo Image caption People braved the weather to pay their respects at a vigil in Trafalgar Square

Mr Cameron told the French people: "Your pain is our pain, your fight is our fight."

He has also called French President Francois Hollande to express his condolences.

Mr Hollande said the attacks were an "act of war" by IS.

In other developments:

The Foreign Office said people with concerns about British relatives or friends in Paris should call 0207 008 0000.

A 41-year-old man from France was detained by police after a suspected firearm was discovered at Gatwick Airport. The airport's North Terminal was evacuated but has since re-opened.

The London School of Economics tweeted that one of its former students, Valentin Ribet, had been killed in Paris. He was a French national.

London's Tower Bridge, the National Gallery, the London Eye and Wembley Stadium are being lit up in the colours of the Tricolour. The same has been done to The Usher Hall in Edinburgh, Glasgow's Hydro and Edinburgh Castle.

Flags are being 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.

At the scene in Trafalgar Square

Image copyright Mario Cacciottolo

BBC correspondent Daniel Boettcher

Trafalgar Square is almost full - hundreds of people have gathered for the vigil, some bringing flowers and candles.

The blue, white and red colours of the French flag are projected on the walls of the National Gallery, painted on people's faces and also illuminate the fountains in the square.

One young woman is carrying a placard which reads "Paris is Life" and another sign says "Nous Sommes Paris".

A short time ago there was applause across the square before the crowd sang La Marseillaise.

UK police said there would be strengthened policing at ports, and more officers at public events in the coming days.

Metropolitan Police Commissioner Sir Bernard Hogan-Howe said police would "urgently review" tactics to deal with firearms attacks in light of the Paris killings.

"The scale of the attacks and the range of weaponry used by the terrorists are a serious cause for concern," he added.

Image caption Belfast City Hall was also lit up

Eyewitnesses: 'People started screaming'

Image copyright AFP

Nottingham student Hanna Corbett has described the moment attackers in Paris stormed into the Bataclan concert hall during a gig by the Eagles of Death Metal.

"It was towards the end of the concert and it sounded like firecrackers or fireworks," she said.

"People started screaming and the lead singer looked startled and he ran off-stage. Then the lights went on and then everyone dropped to the floor.

"[People were] crawling over each other, just trying to climb out of fire exits. It was pretty inhumane."

Hanna was with her friend Jack, also a student in Nottingham, who said he saw one of the gunmen and dropped to the floor.

He said: "We were dealt a pretty lucky hand to have been so close to it and get out pretty much unscathed."