Dozens of people are believed to have been killed after a lorry ploughed into a Bastille Day crowd in Nice.
Footage showed crowds fleeing in terror from the scene in the French city on the Mediterranean coast on Thursday night.
Eyewitnesses reported hearing gunshots and pictures on social media showed armed police with weapons trained on a badly damaged white truck.
Nice mayor Christian Estrosi, who was at the celebration when it happened, tweeted: "Dear people of Nice, the driver of a truck seems to have left dozens dead. Stay for now in your home. More info to come."
French TV channel BFM said President Francois Hollande was returning to Paris from the city of Avignon to hold a crisis meeting at the Interior Ministry.
Colin Srivastava told BBC News: "We were basically sitting just in front of the Old Town in Nice and saw several hundred people running towards us looking panic stricken.
"We tried to ask a few of them what the hell was going on and finally got one that said, 'You need to go, the police have told us to run'.
"Just around about the base of the hill where the castle is in Nice the police came running along and said, 'Run now'.
"We had absolutely no idea what was going on, to be honest with you.
"When we got down into the port in Nice we were told by a few people who'd obviously run faster than we had that there was the story of a lorry that had gone into the people, basically cannoned into the crowd, and there were also shots fired, which is something we didn't hear about until just now on French news."
Bastille Day is France's national day, marking the start of the revolution in 1798 when the prison of the same name was stormed by protesters against the autocratic rule of King Louis XVI.
An eyewitness has said he saw a lorry driver slam his vehicle into the crowd.
Wassim Bouhlel, a Nice native who spoke near Nice's Promenade du Paillon, said he saw a lorry drive into the crowd and the man then emerged with a gun and started shooting.
"There was carnage on the road," Bouhlel said. "Bodies everywhere."
The Gendarmerie Nationale - one of two national police forces of France - tweeted: "Emergency operation in progress. Keep calm and avoid downtown area. Follow the official accounts to be informed."
The lorry ploughed through the crowd on Nice's Promenade des Anglais, which is famed as a landmark and runs for some four miles. Below it lies a rocky pebble beach.
It is a popular walking and cycling route and was busy with revellers celebrating and watching fireworks.