More than 70 people are reported dead after a lorry attack on a Bastille Day celebration in Nice.
Chaos erupted on the streets of the Mediterranean city on Thursday night after the lorry ploughed into crowds who had gathered to celebrate the French national day.
Nice regional president Christian Estrosi, who was at the celebration when the carnage happened, said: "This is the worst Nice drama of history for more than 70 victims have already reported.
"We are terrified and we want to present to all the families our sincere condolences.
Christian Estrosi, president of the region, said the truck in Nice was loaded with arms and grenades.
Regional chief Eric Ciotti told France Info radio from the scene that it was a "scene of horror".
Nice prosecutor Jean-Michel Prette said bodies were strewn about along the roadway.
Interior Ministry spokesman Pierre-Henry Brandet told BFM TV: "It's going to be a very high toll."
Mr Brandet said no hostages had been taken and the driver of the lorry had been "neutralised", adding that authorities were investigating if he acted alone.
Pictures and footage shown on social media showed crowds fleeing in terror from the scene on the Promenade des Anglais, which was busy with revellers celebrating and watching fireworks.
Eyewitnesses reported hearing gunshots and pictures on social media showed armed police with weapons trained on a badly damaged white lorry with what appeared to be bullet holes in the windscreen.
French TV channel BFM said the death toll had reached 60 and that President Francois Hollande was returning to Paris from the city of Avignon to hold a crisis meeting at the Interior Ministry.
A Downing Street spokesman said: "The Prime Minister is being kept updated on reports coming in from Nice.
"We are shocked and concerned by the scenes there. Our thoughts are with all those affected by this terrible incident on what was a day of national celebration.
"The FCO are in touch with the local authorities to seek more information and we stand ready to help any British nationals and to support our French partners."
Mr Estrosi 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."
Sylvie Toffin, a local authority press officer, said: "It's an attack."
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.
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.
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."
Former French President Nicolas Sarkozy tweeted: "Deep emotion and infinite sadness at the Nice attack. Solidarity with the Niçois and the inhabitants of the Alpes-Maritimes."