Gallery: Horrific images emerging following Nice attack
More than 70 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.
The Gendarmerie Nationale 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.
A Foreign Office spokesman said: "We are in touch with the local authorities and seeking more information following an apparent attack on Bastille Day celebration in Nice. If you are in the area follow the instructions of the French authorities."
France is still on alert after the Paris terror attacks on November 13, in which 130 people were killed at sites including the Bataclan Theatre and Stade de France.
French president Francois Hollande vowed at the time to destroy the "army of fanatics" responsible for the terrorist attacks.
And in January 2015 the city was rocked by the Charlie Hebdo attack, when 12 people were killed after gunmen stormed the offices of the satirical magazine.
The Foreign Office said it was in touch with local authorities in Nice and was seeking more information following "an apparent attack on Bastille Day celebration in Nice".
It advised anyone in the area to follow the instructions of the French authorities.