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 23:00 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.