Counter-terrorist investigators are leading the probe into the attack, local media said, and a local government official said weapons and grenades were found inside the truck.
Police shot and killed the driver, who drove the 25-tonne, unmarked truck for well over 100 metres along the famed Promenade des Anglais seafront, slamming into a mass of spectators late on Thursday evening, regional government official Sebastien Humbert toldFrance Info radio.
The man had opened fire on the crowd, local government chief Christian Estrosi told local media, also citing the discovery of weapons and grenades after the driver was killed.
“It’s a scene of horror,” a local member of parliament, Eric Ciotti, told France Info.
He said the truck sped along the pavement fronting the Mediterranean, before being stopped by police after “mowing down several hundred people.”
“People went down like ninepins,” Jacques, a restaurant owner on the seafront Promenade des Anglais, told France Info.
Addressing the nation on television after an emergency meeting, French President Francois Hollande said that the state of emergency in place since November attacks that killed 130 in Paris would be extended by three months.
“There’s no denying the terrorist nature of this attack of yet again the most extreme form of violence,” the President said.
Since Islamic State attacks last year, major public events in France have been guarded by troops and armed police, but it appeared to have taken some minutes to halt the progress of the deadly truck in Nice.
Nice-Matin quoted officials as saying 42 people were in critical condition. Many others were less seriously hurt, including at least one Australian.
Humbert said the driver had not yet been identified. Residents of the city, located 30km (from the Italian border, were advised to stay indoors. There was no sign of any other attack.
Almost exactly eight months ago Islamic State militants killed 130 people in Paris on November 13, the bloodiest in a number of attacks in France and Belgium in the past two years.
On Sunday, France had breathed a sigh of relief as the month-long Euro 2016 soccer tournament ended without a feared attack.
Four months ago, Belgian Islamists linked to the Paris attackers killed 32 people in Brussels.
Police denied rumours on social media of a subsequent hostage-taking. 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.”
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, with a population of some 350,000 and a history as a flamboyant resort but also a gritty metropolis, has seen some 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,” former French intelligence agent and security consultant Claude Moniquet toldFrance-Info, noting the jihadist presence in Nice and the fact that July 14 marks France’s 1789 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 today.
People with concerns for the welfare of Australian family and friends in the region should try to contact them directly, the Department of Foreign Affairs says.
If unable to reach them, Australians should call the 24-hour Consular Emergency Centre on 1300 555 135 or +61 2 6261 3305 if calling from overseas.
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