Children are among those killed after a suspected terror attack on Bastille Day celebrations in Nice, French president Francois Hollande has said.
Mr Hollande said 84 people were dead, including several children, and the country's state of emergency would be extended for a further three months.
A lorry hit crowds who had gathered to celebrate the French national day in the Mediterranean city on Thursday night.
READ MORE: 84 dead in Bastille Day lorry attack in Nice
Mr Hollande said that a military operation was in place that would allow the mobilisation of 10,000 military officers and that police from across the country would be called to assist their colleagues in Nice.
READ MORE: New Foreign Secretary Boris Johnson condemns 'appalling' attack
He said the country intended to tighten borders and would show "real force and military action in Syria and Iraq".
Regional hospitals have been warned to expect an influx of the injured and the president urged citizens to be aware of security over the busy national holiday weekend.
He said: "France has been hit by a tragedy once again. This monstrosity of using a lorry to deliberately kill people, many people, who only came out to celebrate their national day.
READ MORE: People ran from shots as lorry zigzagged through crowds
France is in tears. It is hurting but it is strong, and she will be strong, always stronger than the fanatics who wish to hurt us."
French prosecutors earlier said they had launched a terrorism investigation after the lorry careered through the packed seafront for hundreds of yards as crowds watched fireworks.
READ MORE: Shock and sadness after 'worst tragedy in the history of Nice'
Nice regional president Christian Estrosi said guns and grenades were found in the lorry, whose driver was shot dead by police.
The politician, who was at the celebration when the carnage happened, said: "This is the worst Nice drama of history.
"We are terrified and we want to present to all the families our sincere condolences.
French media reports said that as well as the dead, a further 15 people were critically injured and dozens more wounded.
Nice prosecutor Jean-Michel Prette said bodies were strewn about along the road.
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 lorry driver 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.
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.
The state of emergency initiated after those attacks which has now been extended was due to be lifted later this month.
READ MORE: 84 dead in Bastille Day lorry attack in Nice
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."
New Foreign Secretary Boris Johnson tweeted that he was "shocked and saddened by the appalling events in Nice, and the terrible loss of life".
Mr Estrosi confirmed that a Rihanna concert scheduled for Friday night at the Allianz Riviera stadium in Nice had been cancelled, as had the city's jazz festival which was due to run from Saturday to Wednesday.
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