At least 80 people have been killed and dozens injured after a truck careered along the waterfront in the French city of Nice, ploughing into hundreds of people. Photo: Internet Source

French President Francois Hollande has said several children were among those killed in what he described as a monstrous "terrorist" act.

Video footage shows police trying to stop the lorry before it picks up speed, slamming into revellers gathered along the city's famous waterside Promenade des Anglais for a Bastille Day fireworks display.

The driver zig-zagged along the promenade for some two kilometres (1.3 miles), mowing people down along the way, according to Nice prosecutor Jean-Michel Pretre.

Several officials say he then emerged from the vehicle and started shooting, sending hundreds of terror-stricken men, women and children running in terror.

"We almost died. It was like hallucinating... (the lorry) zigzagged," 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."

"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."

Nice Matin journalist Damien Allemand, who was at the waterside, said the fireworks display had finished and the crowd had got up to leave 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."

© Sky News Attack in Nice

Graphic footage from the scene shows an endless succession of bodies left strewn along the city's landmark promenade in the lorry's wake.

Screams and cries can be heard as dazed victims and witnesses lean over the bodies, looking for survivors.

Items of clothing and shoes are scattered across the blood-stained pavement.

French officials say the driver was shot dead by police at the scene, with images showing the windscreen of the 25-tonne lorry riddled with bullet holes.

Authorities are investigating if he acted alone following media reports that a possible accomplice had fled the scene.

Several weapons, guns and grenades were found inside the vehicle, Christian Estrosi, president of the region, has confirmed.

Identity papers of a French-Tunisian were also discovered, according to a police source quoted by news agency AFP. The papers indicate the man is a 31-year-old resident of Nice.

There has been no claim of responsibility, however, the French president has confirmed 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," he said.

He pledged to strengthen France's role in Syria and Iraq and extended the nation's state of emergency - imposed after November's attacks in Paris - for a further three months.

Nice's famous luxury Negresco hotel, meanwhile, has been turned into a makeshift ward, with dozens of frightened and injured victims gathered in the marble lobby.

The Hotel Negresco has been turned into a makeshift hospital Pic: @AlbanMikoczy

The hashtag #RecherchesNice has been started for people looking for loved ones.

Graphic images from the scene show bodies strewn across the city's landmark promenade, which runs some four miles along the waterfront.

"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 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.

Authorities in Nice told local residents to stay in their homes as they conducted further operations, though there was no sign of any other threats.

The Gendarmerie Nationale tweeted: "Emergency operation in progress. Keep calm and avoid downtown area. Follow the official accounts to be informed."

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.

France is currently under a state of emergency following the Islamic State attacks in Paris in November that left 130 dead.