Dozens dead following Nice attack
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."
An eyewitness has said he saw a lorry driver slam his vehicle into a crowd of Bastille Day revellers in the French resort city of Nice and then emerge shooting, killing many.
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.
French President Francois Hollande, who was in the south of France at the time, had hours earlier said a state of emergency put in place after the Paris attacks in November would not be extended when it was due to expire 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.
Irish citizens in #Nice who need to contact the Embassy can do so at 0144176700. Stay safe and follow instructions of local authorities.— Irish Embassy Paris (@IrlEmbParis) July 14, 2016
The Irish Embassy in Paris have advised any Irish tourists in Nice to contact them on 0033-144176700.
They added: "Stay safe and follow instructions of local authorities."