NICE, France (CBS News) -- A truck drove on to the sidewalk and plowed through a crowd of Bastille Day revelers who'd gathered to watch fireworks in the French resort city of Nice late Thursday in what some officials and eyewitnesses described as a deliberate attack.
At least 75 people were killed and dozens hurt, according to local officials.
French network BFM TV said the victims were pedestrians on the famed Promenade des Anglais along the Nice waterfront.
Sylvie Toffin, a press officer with the local prefecture, said the truck "hit several people on a long trip" down the sidewalk near Nice's Palais de la Méditerranee, a building which fronts the beach. Some estimates say the truck went over a mile through the crowd before being stopped.
Toffin confirmed the incident was deliberate, saying, "It's an attack."
The Paris prosecutor's office said it had open a terrorism investigation into the crash.
The mayor of Nice, Christian Estrosi, said on Twitter the truck "seems to have killed dozens of people." Estrosi told residents to stay inside their homes.
Estrosi said the truck was loaded with arms and grenades, and that the driver was shot dead by police.
The mayor told BFM TV 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 behavior." He added that "the truck was loaded with arms, loaded with grenades."
Wassim Bouhlel, a Nice native who spoke to the Associated Press near Nice's Promenade du Paillon, said that he saw a truck drive into the crowd and then witnessed the man emerge with a gun and start shooting.
"There was carnage on the road," Bouhlel said. "Bodies everywhere."
Video posted on social media showed chaos as people ran from the crash scene.
Municipal police in Nice urged residents to remain calm and stay indoors.
"It's a scene of horror," local member of parliament Eric Ciotti told France Info, saying the truck had sped along the pavement fronting the Mediterranean, "mowing down several hundred people."
French President Francois Hollande was heading back to Paris early Friday for crisis talks, his office told AFP.
"He is coming back to Paris and will go directly to Place Beauvau to the crisis cell," the presidency said. Hollande had been in the southern city of Avignon on a private visit.
While the incident in Nice appears to be deliberate, U.S. law enforcement has no official analysis of what happened, CBS News Justice and Homeland Security correspondent Jeff Pegues reported.
July 14th in France is marked with commemorations of the Storming of the Bastille fortress in 1789 at the start of the French Revolution. It is celebrated similarly to July 4th in the U.S.
There was no word who or which group was behind the attack. Thursday's attack comes almost exactly eight months after the November 2015 Paris attacks that killed 130 people, the worst terror attack in France's history.
The White House released a statement from President Obama on Twitter, condemning "what appears to be a horrific terror attack..."
At this time it is too early to know who is involved, the law enforcement officials said.
Almost exactly eight months ago, ISIS militants killed 130 people in a series of attacks in Paris.
The hashtag #PrayForNice quickly soared to the top of Twitter's trending section, with thousands of people expressing their thoughts on social media.
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