Eyewitnesses have told how crowds fled after a truck rammed into revellers celebrating Bastille Day in Nice.
Pictures and video have also been posted from people who were present as the attack unfolded in southern France.
Journalist Damien Allemand said he was at the Nice waterside and that the fireworks display had finished when the crowd heard a noise and cries.
"A fraction of a second later, an enormous white truck came along at a crazy speed, turning the wheel to mow down the maximum number of people," the Nice Matin journalist said.
"I saw bodies flying like bowling pins along its route. Heard noises, cries that I will never forget."
Allemand said people took shelter in a nearby restaurant, where he continued to hear people shouting for missing family members.
"This evening, it was horror," he said, adding that he saw the bodies of victims on the street.
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'."
The region's president Christian Estrosi told a TV station that "the driver fired on the crowd, according to the police who killed him".
Estrosi said the truck was driven by someone who appeared to have "completely premeditated behaviour".
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.