At least 84 people have died, including children, after a lorry slammed through a crowd celebrating Bastille Day in the southern French city of Nice.
The driver ploughed on for 2km on the Promenade des Anglais at about 2300 local time, before being shot dead by police.
President Francois Hollande said the attack was of "an undeniable terrorist nature".
A state of emergency, in place since November's Paris attacks, has been extended by three months.
France had been on high alert since those attacks in which 130 people died and hundreds were wounded.
"France is badly hit," Mr Hollande said, adding that "we need to do everything we can to fight against" such attacks.
"All of France is under the threat of Islamic terrorism," he said, adding that several children were among the dead.
The president added that "operational reserves" would be deployed to support the army and security forces across the country, with particular focus on the borders.
About 50 people were injured, 18 of them critically, in the incident on Thursday, which was Bastille Day - France's national day.
Prosecutor Jean-Michel Pretre said the lorry drove 2km (1.2 miles) through a large crowd, the AFP news agency reported.
One image on Twitter showed about a dozen people lying on the street.
Interior Ministry spokesman Pierre-Henry Brandet denied earlier reports of hostage situations and said the driver of the lorry had been "neutralised".
He added that officials were investigating whether the driver acted alone.
No group has so far claimed responsibility, however prosecutors said the inquiry would be handled by anti-terror investigators.
Some reports spoke of shots being exchanged between police and the occupants of the lorry but these have not been confirmed.
Social media video showed people running through the streets in panic following the incident.
A journalist with the Nice Matin newspaper reported from the scene that there was "a lot of blood and without doubt many injured".
Another witness, Roy Calley, told the BBC that there were "thousands of people on the promenade" when the incident happened.
"The police have completely taken over the city, the promenade has now been closed down. Everybody was physically pushed away from the site and told to get back in no uncertain terms by the police."
"I live 200m from the promenade and it took nearly one hour 30 minutes to get back to my flat because all the roads have been closed down."
US President Barack Obama condemned "in the strongest terms" what he said appeared to be "a horrific terrorist attack in Nice", the White House said.
The president had been briefed about the situation "and his national security team will update him, as appropriate", National Security Council spokesman Ned Price said.
British Foreign Secretary Boris Johnson said he was "shocked and saddened by the appalling events in Nice, and the terrible loss of life".
European Council President Donald Tusk condemned the attack, saying "it's a tragic paradox that the subject of this attack were people celebrating liberty, equality and fraternity".
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