Christian Estrosi, a leading regional politician in the area, said on Twitter: “This is the worst drama in the history of Nice.”

French Interior Minister Bernard Cazeneuve said 80 people had been killed, and 18 were in a “critical condition”. Hollande said “several children” were among the dead.

Christian Estrosi, the president of the local region said of the truck driver: “At the moment that he was shot dead by police, he had fired several times.”

Authorities said they found identity papers belonging to a 31-year-old French-Tunisian citizen in the 19-tonne truck.

A source close to the investigation said an “inactive” grenade was found inside the 19-tonne truck, as well as “several fake rifles”.

Regional lawmaker Eric Ciotti described “terrifying images, a scene of absolute horror, with many children among the dead”.

“Families who were sharing a moment of leisure were targeted. It was a symbolic day in a symbolic place. That ... motivated the attack,” he said.

Hollande was quick to decry the incident as terrorism.

“France has been struck on the day of her national holiday ... the symbol of liberty,” he said. “We show our solidarity towards the victims and their families. All means are being deployed to help the wounded. France as a whole is under the threat of Islamic terrorism. We have to demonstrate absolute vigilance and show determination that is unfailing.

“I have decided that the state of emergency which was supposed to end on 26 July will be extended by three months.”

Photos showed the white truck riddled with bullet holes. “An individual drove a truck into the crowd. He was killed by police,” said interior ministry spokesman Pierre-Henry Brandet. Social media showed casualties lying in the street, some being tended by paramedics.

The incident on Thursday night comes as France remains badly shaken by November 2015 attacks which killed 130 people at the Bataclan concert hall in Paris and nightspots across the city.

Describing it as a “major criminal attack”, Sebastien Humbert, the sub-prefect of the Alpes-Maritimes region, told BFMTV the driver had left “around a hundred injured”.

Police and ambulances went to the scene and authorities from the local Alpes-Maritimes prefecture urged residents to stay indoors.

The white truck drove at high speed onto the famed Promenade des Anglais as people were leaving after the annual Bastille Day celebration display.

“We saw people hit and bits of debris flying around,” said an AFP reporter who was in the crowd.

“I had to protect my face from flying debris.”

The correspondent described the moment that the large white truck plunged into crowds of people who had come to the bustling Mediterranean beachfront to celebrate.

Terrified people screamed and scattered as it veered down the promenade, where adults and children were walking towards the famous Negresco Hotel.

Nice prosecutor Jean-Michel Pretre said the truck drove two kilometres through the crowd, mowing people down in what one local official has called a “major criminal attack”.

“For a big truck like that to get actually onto the promenade and then to go in a fairly straight line along there, looked to me like a very deliberate act,” the reporter said.

“It was about 100 metres from me and I had a few seconds to get out of the way.”

Police could be seen surrounding the truck in the early hours of Friday, its windscreen riddled with bullet holes, under the lights of the Negresco Hotel.

Australian Emily Watkins said she was less than 50 metres away from the incident and saw the truck on the promenade but did not realise what was happening.

“There was a lot of confusion. I don’t actually remember seeing the truck moving,” she told the Australian Broadcasting Corporation.

“There was a lot of screams coming from ahead of us where the truck was, and people just running towards us and without really knowing what was going on we turned and ran as well.

“As we were running away we could hear what I thought at the time were more flares or fireworks.

“People were tripping over and trying to get into hotel lobbies and restaurants or car parks or anywhere they could to get away from the street.”

The incident came just hours after France announced that a state of emergency declared after last November’s attacks would come to an end later this month.

Speaking on the national July 14 holiday, President Francois Hollande said the decree after the November 13 attacks would not be renewed beyond July 26, because a law bolstering security in France was adopted in May.

Nice city mayor Christian Estrosi urged residents to stay in doors.

“The driver of a van appears to have killed dozens. Stay in your homes for now. More info to follow,” Estrosi wrote on Twitter.

Agence France-Presse, The Guardian, Associated Press and Reuters