NICE, France -- A truck loaded with weapons and hand grenades drove onto a sidewalk for more than a mile late Thursday, plowing through Bastille Day revelers who had gathered to watch fireworks in the French resort city of Nice.
At least 77 people were killed before police killed the driver, authorities said.
Nice prosecutor Jean-Michel Pretre described a horrific scene, with bodies strewn about along the roadway.
Sylvie Toffin, a press officer with the local prefecture, said the truck ran over people on a "long trip" down the sidewalk near Nice's Palais de la Mediterranee, a building that fronts the beach.
Wassim Bouhlel, a Nice native, said he saw a truck drive into the crowd.
"There was carnage on the road," he said. "Bodies everywhere."
The Paris prosecutor's office opened an investigation for "murder, attempted murder in an organized group linked to a terrorist enterprise."
The ranking politician of the Alpes-Maritime department that includes Nice said the truck plowed into the crowd over a distance of 1.2 miles, killing 77 people and wounding 50.
Many of those on the ground were in shorts and other summer clothing.
Eric Ciotti said on BFM TV police killed the driver, "apparently after an exchange of gunfire."
The president of the Provence Alpes Côte d'Azur regional council, which includes Nice, said the truck was loaded with arms and grenades. Christian Estrosi told BFM TV "the driver fired on the crowd, according to the police who killed him."
Bouhlel said he witnessed the man emerge with a gun and start shooting.
Images being broadcast across French media showed revelers running for their lives down Nice's palm tree-lined Promenade des Anglais, the famous seaside boulevard named for the English aristocrats who proposed its construction in the 19th century.
Video footage showed men and women -- one or two pushing strollers -- racing to get away from the scene.
And, in what appeared to be evidence of a gun battle, photos showed a truck with at least half a dozen bullet holes punched through its windshield.
It was not immediately clear who would have been behind an attack, but France has seen a spate of dramatic assaults recently by jihadist groups, including the Islamic State group.
President Barack Obama condemned what he said "appears to be a horrific terrorist attack."
"Our thoughts and prayers are with the families and other loved ones of those killed," he said.
European Council president Donald Tusk said it was a "tragic paradox" the victims of the attack in Nice were celebrating "liberty, equality and fraternity" -- France's motto -- on the country's national day.
France's ambassador to the United States, Gerard Araud, characterized the events as a "terrorist attack."
Writing online, Nice Matin journalist Damien Allemand, who was at the waterside, said the fireworks display had finished, and the crowd had gotten up to leave when they 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," he said."I saw bodies flying like bowling pins along its route.
"Heard noises, cries that I will never forget."
Graphic footage showed a scene of horror up and down the Promenade, with broken bodies splayed out on the asphalt, some of them piled near one another, others bleeding out onto the roadway or twisted into unnatural shapes.
"Help my mother, please!" one person yelled out amid a cacophony of screaming and crying.
A pink girl's bicycle was seen briefly, overturned by the side of the road.
The origin and authenticity of the footage could not be verified immediately.
Kayla Repan of Boca Raton, Florida, was among the hundreds gathered on the promenade to watch fireworks.
"The whole city was running. I got extremely frightened and ran away from the promenade," she said. "It was chaos."
Associated Press writers Elaine Ganley in Paris and Naomi Koppel in London contributed to this report.
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