A gunman at the wheel of a heavy truck ploughed into a crowd celebrating Bastille Day in the French city of Nice on Thursday, killing at least 84 people and injuring scores more in what President Francois Hollande called a terrorist act.

The attacker, identified by a police source as a 31-year-old Tunisian-born Frenchman, also opened fire before police shot him dead. He had been known to the police for common crimes but not to the intelligence services, the source said.

Interior Minister Bernard Cazeneuve said 18 of the injured were in a critical condition after the 25-tonne truck zigzagged along the seafront Promenade des Anglais as a fireworks display marking the French national day ended just after 10:30 p.m. (2030 GMT).

The attack, which came eight months and a day after Islamic State gunmen and suicide bombers struck Paris on a festive Friday evening, seemed so far to be the work of a lone assailant.

Hollande said in a pre-dawn address that he was calling up military and police reservists to relieve forces worn out by a state of emergency begun after the militant group killed 130 people in the French capital in November.

Only hours earlier Hollande had announced the state of emergency would be lifted by the end of July, but the president said that following the attack, in which several children were killed, it would now be extended by a further three months.

"France is filled with sadness by this new tragedy," he said. "There's no denying the terrorist nature of this attack of yet again the most extreme form of violence."

Officials said hundreds were hurt as the driver wove along the seafront, knocking them down "like skittles". A local government official said weapons and grenades were found inside the unmarked articulated truck.

Dawn broke on Friday with the pavements smeared by dried blood, while smashed children's strollers, an uneaten baguette and other debris were strewn about the Mediterranean seaside promenade. Small areas were screened off at regular intervals. What appeared to be bodies covered in blankets were visible through the gaps.

The scene appeared to confirm what one city official said during the night - that the truck drove a full 2 km (1.5 miles)along the promenade after mounting the kerb.

The truck, a rental vehicle according to local officials, was still where it came to rest, its windscreen riddled with bullets.

Hollande called the tragedy on the day that France marks the 1789 revolutionary storming of the Bastille prison in Paris an attack on liberty by fanatics who despised human rights.

France would, nonetheless, continue its air operations against Islamic States in Syria and Iraq.

Police were trying to establish whether the driver might have had any accomplices in a city with a reputation for Islamist activism. There had been no claim of responsibility on Friday morning.

German Chancellor Angela Merkel condemned what she called an act of mass murder. "Germany stands at France's side in the fight against terrorism, united with many, many others. I am convinced that, despite all the difficulties, we shall win this fight," she said at a summit meeting in Mongolia.

"A SCENE OF HORROR"

The truck careered 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," member of parliament 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.

Major events in France have been guarded by troops and armed police since the Islamic State 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.

Police told residents of the city, 30 km (20 miles) from the Italian border, to stay indoors as they conducted further operations, although there was no sign of any other attack.

Hollande also called up former troops and gendarmes after racing back to Paris from the south of France in the wake of the attack.

The Paris attack on November 13 was the bloodiest among a number 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 Islamists 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.

US 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".

On social media, Islamic State 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."

Officials have warned in the past of the risk of Islamist attacks in the region following the Paris and Brussels attacks. Reverses for Islamic State in Syria and Iraq 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.

Nice, a city of 350,000, has a history as a flamboyant, aristocratic resort but is also a gritty metropolis. It has seen dozens of its Muslim residents travel to Syria to fight, a path taken by previous Islamic State attackers in Europe.

"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.

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." - Reuters