At least 84 people have died and dozens hurt after a terrorist drove a truck through crowds along Nice’s packed waterfront.
Several children were among the dead after the driver mowed down people celebrating Bastille Day along the famous waterside promenade.
Video footage shows police trying to stop the lorry before it picks up speed, slamming into revellers at the end of a fireworks display.
Witnesses said the driver swerved from side to side to kill as many people as possible along 1.2 miles of the promenade.
He eventually emerged from the vehicle to shoot indiscriminately before being shot dead by police.
Several guns and grenades were found inside the vehicle, Christian Estrosi, president of the region, has confirmed.
French President Francois Hollande described the attack as a “monstrous terrorist act”.
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Graphic footage shows a horrifying succession of bodies strewn along the city’s landmark promenade in the lorry’s wake.
“We almost died. It was like hallucinating… (the lorry) zig-zagged," an eyewitness told i-Tele.
"You had no idea where it was going. My wife… a metre away … she was dead.
"The lorry ripped through everything. Poles, trees, we have never seen anything like it in our lives.
"Some people were hanging on the door trying to stop it.”
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Nice-Matin journalist Damien Allemand said the fireworks were over and people were leaving when they heard a noise and cries.
“A fraction of a second later, an enormous white truck came along at a crazy speed, turning the wheel to mow down the maximum number of people,” he said.
“I saw bodies flying like bowling pins along its route. Heard noises, cries that I will never forget.”
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Screams and cries can be heard as dazed witnesses lean over the bodies, looking for survivors.
Items of clothing and shoes are scattered across the blood-stained pavement.
Images show the windscreen of the 25-tonne lorry riddled with bullet holes.
British 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.
"Can’t stop shaking. Hate that my boys had to experience this. Why did I take them.
"Why did they do this and why the **** is this happening!”
There has been no claim of responsibility.
However, a police source has identified the attacker as a 31-year-old Franco-Tunisian born in Tunisia.
Though unconfirmed, this tallies with the profile of a man whose identity papers were reportedly discovered inside the vehicle.
A police source quoted by news agency AFP said the papers belonged to a resident of Nice.
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In a live TV address, the French president said there is “no denying the terrorist nature of this attack”.
“I express in the name of the nation, our tears, solidarity, with the victims and the families,” Mr Hollande said.
He pledged to strengthen France’s role in Syria and Iraq.
He also extended the nation’s state of emergency - imposed after November’s attacks in Paris - for a further three months.
At least 18 people remain in a critical condition, with hospitals in the area appealing for blood.
Laurence Marie, who works at Lenval paediatric hospital, has said “many” children are undergoing serious operations there.
Nice’s famous luxury Negresco hotel, meanwhile, has been turned into a makeshift ward.
Dozens of frightened and injured victims gathered in the marble lobby.
The hashtag #RecherchesNice has been started for people still looking for loved ones.
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“This is the worst disaster in the history of Nice as more than 70 people have already lost their lives, killed by the driver as he fired at the crowds ahead of him,” Mr Estrosi said in a statement.
“We are terrified and we want to express our sincere condolences to the families involved.
"Today, we must act. We are currently in touch with the State authorities who are dealing with this crisis.”
The Paris prosecutor’s office has opened an investigation for “murder, attempted murder in an organised group linked to a terrorist enterprise”.
The probe is being handled by France’s intelligence agency and judicial police.
The Foreign Office says it is in touch with local authorities in Nice and has advised anyone in the area to follow the instructions of French authorities.
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The city of Marseille has cancelled its fireworks show on Friday after the attack.
In Britain, Labour leadership hopeful Owen Smith announced that he would be cancelling his campaign launch.
The attack comes days after France breathed a sigh of relief after hosting the Euros 2016 without incident.
The tournament took place under heavy security following the Paris attack that left 130 dead.
An emergency contact number has been set up for anyone who may have loved ones caught up in the attack in Nice: 0033493722222.