French president Francois Hollande has said that "France is in tears" after at least 84 people - including several children - were killed in a suspected terror attack on Bastille Day celebrations in Nice.
A further 18 people are in a critical condition as Hollande said the country's state of emergency, which was due to be lifted this month following the Paris attacks in November last year, would be extended for a another three months.
A lorry, containing guns and grenades, hit crowds who had gathered to celebrate the French national day in the Mediterranean city on Thursday night. The driver was shot dead by police.
Hollande led a wave of tributes from world leaders saying: "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.
"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."
He also 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.
The country borders will be tightened and would show "real force and military action in Syria and Iraq", Hollande added.
Regional hospitals had 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.
In a press conference at the scene, French interior minister Bernard Cazeneuve said that the identification of the driver was in progress, refusing to confirm reports that an ID card was found after police shot him dead, and added that more information would come from the prosecutor's office.
"We are in a war with terrorists who want to strike us at any price and in a very violent way," the minister said.
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.
Nice regional president Christian Estrosi said guns and grenades were found in the lorry. 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.
Nice prosecutor Jean-Michel Prette said bodies were strewn along the road. Pictures and footage shown on social media showed crowds fleeing 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 bullet holes in the windscreen.
A Foreign Office spokeswoman said: "We are deeply concerned about reports of an incident in Nice and are in touch with the local authorities to seek more information. Our thoughts are with those affected and we stand by to help any British nationals."
A Rihanna concert scheduled for Friday night at the Allianz Riviera stadium in Nice has been cancelled, as have the city's jazz festival which was due to run from Saturday to Wednesday and some of the Bastille celebrations over the weekend.