A truck mowed through a crowd on a seaside promenade during a Bastille Day celebration in the French city of Nice in what President Francois Hollande called an attack of "a terrorist nature."
On Friday after the attack, Hollande extended a state of emergency for three months and declared three days of national mourning.
Terror, said Prime Minister Manuel Valls, would long plague France. “The threat of terrorism, as we have now been saying for a long time, is weighing heavily on France, and it will continue to do so for a long time yet."
French officials said the attack killed at least 84 people, including children. About 50 people were wounded, 18 of them critically, officials said.
The attack took place during a fireworks display Thursday night marking France's most important nationa holiday. Social media video showed people running through the streets in panic following the attack.
Authorities said the truck driver was shot dead. Police say ID found with the attacker was of a French petty criminal, 31, of Tunisian descent, the AP reported. There was no DNA confirmation.
Interior Ministry spokesman Pierre-Henry Brandet said officials were investigating whether the driver acted alone. There was no immediate claim of responsibility.
"Nobody seems to have much doubt or ambiguity as to what this incident was," Paris-based journalist Vivienne Walt told PRI's The World.
Rebecca Giraud described her restaurant as 20 meters from the attack. Before the attack, she said, "It was really a great night for us ... We were full all night long."
Then, she heard screams and "everybody was running into the restaurant. ... We were so afraid." She told The World that they hid the people upstairs and in the basement, not knowing who was behind the attack. They left the restaurant without cleaning up later, and it was going to be closed on Friday.
"We will try to sleep," Giraud said shortly after 2 a.m., "but it won't be easy tonight."
President Barack Obama, in a statement, condemned the attack and said the United States stood by France.
Lidia Jean Kott, Leo Hornak, Kierran Petersen and Marco Werman of PRI's The World contributed to this report.