At least 84 people, including several children, have died after a terrorist drove a truck through crowds celebrating Bastille Day in Nice.
Eyewitnesses said the killer swerved from side to side to kill as many people as possible as he drove for hundreds of metres along the Promenade des Anglais on the seafront of the city on the French Riviera.
New Foreign Secretary Boris Johnson said a Briton is believed to be among the scores injured.
Speaking to the BBC as he left his home, Mr Johnson said: “ It’s an absolutely appalling incident and there will be ministerial meetings later on today to discuss the implications.
“Clearly this represents a continuing threat. If this is a terrorist incident, as this appears to be, this represents a continuing threat to us in the whole of Europe and we must meet it together.
“The only information that I have is that there is one UK national who is injured.”
Eyewitnesses in the resort said they assumed the driver had lost control as panic broke out.
But reports from the south of France suggested the driver - believed to be a 31-year-old French-Tunisian man - then produced a gun before being shot dead by police.
Witness Eric Ciotti told Europe 1 radio of the horrifying moments people fled to safety - with some running into the sea in a desperate attempt to flee.
He added: “A person jumped on to the truck to try to stop it.
“It’s at that moment that the police were able to neutralise this terrorist. I won’t forget the look of this policewoman who intercepted the killer.”
Egyptian Nader El Shafei told the BBC he saw the driver “looking very nervous”.
He said: “I kept yelling at him, waving with my hands to stop and trying to tell him that there is a lot people under his truck - dead already. But he did not give any attention to anyone outside the truck.
“And suddenly I saw him picking up something like a cellphone. I thought he would call the ambulance for the accident but it seemed that I was wrong, because he just picked up his gun and he started to shoot the police.
“Just when they (the police) arrived they just felt something was wrong so they kept yelling at him and when he did not step out - they saw him from the window taking his gun out.
“They knew that would be a gun shooting so they just killed him right away - they did not wait to negotiate or something, they just opened fire on him.”
Christian Estrosi, the regional president in Nice, said at least 10 children were among the dead, and the death toll is expected to rise.