WARNING: This story contains graphic video and images.

A "terrorist" gunman killed 84 people and wounded scores when he drove a heavy truck at high speed into a crowd that had watched Bastille Day fireworks in the French Riviera city of Nice late on Thursday, officials said.

France's Interior Ministry said 84 people died and 18 were in a critical condition. Many more were also wounded in the attack along the famed seafront Promenade des Anglais as the fireworks ended just after 10:30 p.m.

The driver also opened fire before police shot him dead.

At least 10 children are among the dead, said Christian Estrosi, the regional president in Nice.

In a pre-dawn address to the nation, President Franç​ois Hollande said he was calling up military and police reservists to relieve forces worn out by an eight-month state of emergency begun after the Islamic State of Iraq and Syria (ISIS) militant group killed 130 people in Paris. The state of emergency was extended by three months.

France is afflicted, but is strong and will always be stronger than the fanatics who want to strike her, says François Hollande 6:07

"France is filled with sadness by this new tragedy," Hollande said, noting several children were among the dead in what he said he had no doubt was an act of terrorism.

France to keep fighting ISIS

He called the carnage, which came as France celebrated the anniversary of the 1789 revolutionary storming of the Bastille, an attack on liberty by fanatics who despised human rights.

France would, nonetheless, continue military operations in Syria and Iraq, he said.

French soldiers advance on the street after the attack in Nice (Eric Gaillard/Reuters)

Counter-terrorist investigators were seeking to identify the driver. A local government official said weapons and grenades were found inside the 25-tonne, unmarked articulated truck.

Officials said hundreds were hurt as the driver wove along the seafront, knocking them down "like skittles."

People flee after a truck plows into crowd in Nice, France 1:21

The attack, which came eight months and a day after ISIS gunmen and suicide bombers struck the French capital on a festive Friday evening, seemed so far to be the work of a lone assailant. Police were working to establish what accomplices he may have had in a city with a reputation for Islamist extremism. There had been no claim of responsibility made almost six hours after the attack.

'People went down like ninepins'

The truck careened for hundreds of metres along the front facing the Baie des Anges (Bay of Angels), slamming into families and friends listening to an orchestra or strolling above the beach towards the grand, century-old Hotel Negresco.

"It's a scene of horror," MP Eric Ciotti told France Info radio, saying the truck "mowed down several hundred people."

Jacques, who runs Le Queenie restaurant on the seafront, told the station: "People went down like ninepins."

Bystander Franck Sidoli, who was visibly shocked, said: "I saw people go down."

"Then the truck stopped, we were just five metres away. A woman was there, she lost her son. Her son was on the ground, bleeding," he told Reuters at the scene.

Photographs of the truck show its windshield starred by a score of bullets and its radiator grille destroyed.

Forensic officers stands near a truck with its windscreen riddled with bullets, that plowed through a crowd of revellers who'd gathered to watch the fireworks in the French resort city of Nice. (Claude Paris/Associated Press)

Major events in France have been guarded by troops and armed police since the ISIS attacks last year, but it appeared to have taken many minutes to halt the progress of the truck as it tore along pavements and a pedestrian zone.

State of emergency

Police told residents of the city, 30 km from the Italian border, to stay indoors as they conducted further operations, although there was no sign of any other attack.

Hours earlier, Hollande had said the state of emergency would end in two weeks. He has now extended it by three months, calling up former troops and gendarmes after racing back to Paris from the south of France in the wake of the attack.

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ISIS militants killed 130 people in Paris on Nov. 13, the bloodiest in a number of attacks in France and Belgium in the past two years. On Sunday, a weary nation had breathed a collective sigh of relief as the month-long Euro 2016 soccer tournament across France ended without a feared attack.

Four months ago, Belgian extremists linked to the Paris attackers killed 32 people in Brussels.

Vehicle attacks have been used by isolated members of militant groups in recent years, notably in Israel, as well as in Europe, though never to such devastating effect.

Obama, Trudeau speak out

U.S. President Barack Obama said in a statement: "On behalf of the American people, I condemn in the strongest terms what appears to be a horrific terrorist attack in Nice, France, which killed and wounded dozens of innocent civilians."

The United Nations Security Council said it "condemned in the strongest terms the barbaric and cowardly terrorist attack".

Canadian ​Prime Minister Justin Trudeau also offered his sympathy.

Canadians are shocked by tonight's attack in Nice. Our sympathy is with the victims, and our solidarity with the French people. —@JustinTrudeau

"On behalf of all Canadians, I extend my deepest condolences to the families and friends of those who were killed. We also wish a speedy recovery to the many more that were injured," Trudeau said in a statement.

"Canada and France are the closest of friends, and we stand by the French people as they face this terrible ordeal. We have offered all possible assistance to the French Government.

"Senseless acts like this one are not isolated events, and we will continue to work with our Allies and partners to fight terrorism in all of its forms. We will bring those who are responsible to justice, whether they be the perpetrators, or those involved in funding or organizing such attacks."

On social media, ISIS supporters celebrated the high death toll.

Hiding in terror

One woman told France Info that she and others had fled in terror: "The lorry came zig-zagging along the street. We ran into a hotel and hid in the toilets with lots of people."

Nice-Matin journalist Damien Allemand had been watching the traditional seaside firework display when the truck tore by just as it ended. After taking cover in a cafe, he wrote on his paper's website of what he saw when he came back out on the promenade: "Bodies every five metres, limbs ... Blood. Groans."

"The beach attendants were first on the scene. They brought water for the injured and towels, which they placed on those for whom there was no more hope."

Families with children were among the victims of the attack. (Eric Gaillard/Reuters)

Officials have warned in the past of the risk of ISIS attacks in the region following the Paris and Brussels attacks. Reverses for ISIS have raised fears it might strike again in Europe, possibly again using alienated young men from the continent's Arab immigrant communities whom it has inspired to take up arms against their native countries.

"Neither the place nor the date are coincidental," a former French intelligence agent and security consultant, Claude Moniquet, told France-Info, noting the jihadist presence in Nice and the fact that July 14 marks France's revolution.

"Tragic paradox that the subject of Nice attack was the people celebrating liberty, equality and fraternity," European Council President Donald Tusk said on Twitter.

Tragic paradox that the subject of <a href="https://twitter.com/hashtag/NiceAttack?src=hash">#NiceAttack</a> was the people celebrating liberty, equality and fraternity —@eucopresident

European and Asian leaders pay respect to victims of <a href="https://twitter.com/hashtag/Nice?src=hash">#Nice</a> attack. We stand united against violence and hatred <a href="https://t.co/NU0IRhZd7H">pic.twitter.com/NU0IRhZd7H</a> —@eucopresident

At Nice's Pasteur hospital, medical staff were treating large numbers of injuries.

Waiting for friends who were being operated on, 20-year-old Fanny told Reuters she had been lucky.

"We were all very happy, ready to celebrate all night long," she said. "I saw a truck driving into the pedestrian area, going very fast and zig-zagging.

"The truck pushed me to the side. When I opened my eyes I saw faces I didn't know and started asking for help ... Some of my friends were not so lucky. They are having operations as we speak. It's very hard, it's all very traumatic."