French detectives are tonight trying to piece together the circumstances that left at least 84 people dead and scores injured after a terrorist deliberately drove a lorry into Bastille Day revellers, before being fatally wounded in a stand-off with armed police.
The country’s president Francois Hollande said some 50 people were “between life and death”, while several people are among the missing and a “small number” of Britons are injured.
Prime Minister Theresa May said Britain must redouble its efforts to defeat “brutal” terrorist “murderers”, while police forces across England and Wales have been told to review security at major events over the next week in the wake of the bloodbath.
France has declared three days of national mourning following the atrocity, which comes after attacks in November in Paris in which 130 died and in January 2015 that killed 17.
Eyewitnesses said the Nice attacker - reported to be 31-year-old Mohamed Lahouaiej Bouhlel, a man of French-Tunisian origin who was known to the authorities for anti-social behaviour - swerved from side to side to kill as many people as possible as he drove for a mile along the Promenade des Anglais on the seafront of the city on the French Riviera.
He is said to have pulled a gun from the cab as part of the premeditated attack before being shot dead by police, with people fleeing into the sea in a bid to escape.
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, who celebrated Bastille Day at the French Embassy in London, 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.”
Speaking ahead of a visit to Scotland, Mrs May said she will speak to French president Francois Hollande today and “make clear that the United Kingdom stands shoulder-to-shoulder with France today as we have done so often in the past”.
She said: “If, as we fear, this was a terrorist attack, then we must redouble our efforts to defeat these brutal murderers who want to destroy our way of life.
“We must work with France and our partners around the world to stand up for our values and for our freedom.”
French ambassador to the UK Sylvie Bermann said the massacre in Nice was an attack on liberty, equality and fraternity.
Eyewitnesses in the resort initially thought the commotion was part of a celebratory firework display, but then saw the lorry and assumed the driver had lost control.
Damien Allemand, a journalist with the Nice Matin newspaper, said: “I saw bodies flying like bowling pins in its path. Heard noises, screams that I will never forget.
“I was paralysed. I did not move.”
He said there were lifeless bodies, limbs and blood every few metres.
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.
Mr Hollande led a wave of condemnation from world leaders, saying: “France has been hit by a tragedy once again. This monstrosity of using a lorry to deliberately kill people, many people, who only came out to celebrate their national day.
“France is in tears. It is hurting but it is strong, and she will be strong, always stronger than the fanatics who wish to hurt us.”
He also said a military operation was in place which would allow the mobilisation of 10,000 troops, and that police from across the country would be called to assist their colleagues in Nice.
The country’s borders were being tightened, said the president, as he vowed that France would show “real force and military action in Syria and Iraq”.
In a press conference at the scene, French Interior Minister Bernard Cazeneuve said: “We are in a war with terrorists who want to strike us at any price and in a very violent way.”
Lawyer Harjit Sarang and her children were among those caught up in the terror.
The Londoner tweeted: “Running through crowds in Nice with kids and terrified. Never taking kids to a public event again. Finally back to hotel. Hate this!”
She said it was the “scariest thing ever running through crowds with boys. Got back to hotel and couldn’t get in for people seeking refuge!”
Briton Will Shore told BBC Radio 4: “There was a lot of people screaming, running around and people were kind of being pushed over, I think, from people just being so frightened about what was going on, especially after hearing the gunshots.
“I had to help a couple of people up who were in distress on the floor because everyone was in such a panic.”
Many families with children had gone to the seafront to watch the Bastille Day fireworks and in the aftermath adults were seen trying to comfort and calm terrified youngsters.
A Foreign Office spokeswoman said: “We are deeply concerned about reports of an incident in Nice and are in touch with the local authorities to seek more information.
“Our thoughts are with those affected and we stand by to help any British nationals.”
A Downing Street spokesman said: “The Prime Minister is being kept updated on reports coming in from Nice.
“We are shocked and concerned by the scenes there. Our thoughts are with all those affected by this terrible incident on what was a day of national celebration.
“The FCO are in touch with the local authorities to seek more information and we stand ready to help any British nationals and to support our French partners.”
A Rihanna concert scheduled for Friday night at the Allianz Riviera stadium in Nice has been cancelled, as have the city’s jazz festival, which was due to run from Saturday to Wednesday, and some of the Bastille celebrations over the weekend.
The Association of British Travel Agents (Abta) issued a statement encouraging people to check with their tour operators before heading to France.