An attacker killed at least 75 people and injured scores when he drove a truck at high speed into a crowd watching Bastille Day fireworks in the French Riviera city of Nice late on Thursday, a local politician said.
Counter-terrorist investigators were 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 (yards) along the famed Promenade des Anglais seafront, slamming into a mass of spectators late in the evening, regional government official Sebastien Humbert told France 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 75 people were dead after 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.
Local newspaper Nice-Matin posted photographs of the truck, its windshield starred by a score of bullets and its radiator grille destroyed. 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.
Humbert said the driver was not yet identified. Residents of the city, located 30 km (20 miles) 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 Nov. 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.
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."
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."
Regional government chief Estrosi has warned in the past of the risk of Islamist attacks in the region, following the attacks in Paris and Brussels over the past 18 months.
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 told France-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.
French President Francois Hollande, was in the south of France at the time of the attack but raced back to Paris to the national crisis centre. Hours earlier, Hollande had said that a state of emergency put in place after the Paris attacks in November would not be extended when it expires on July 26.
"We can't extend the state of emergency indefinitely, it would make no sense. That would mean we're no longer a republic with the rule of law applied in all circumstances," Hollande told journalists in a traditional Bastille Day interview.
His interior minister, Bernard Cazeneuve, was expected in Nice overnight, a source in the ministry said. Officials said Hollande would chair a meeting of security chiefs at 9 a.m. (0700 GMT).