DEVELOPING: A large truck loaded with guns and hand grenades drove into a crowd of people who had gathered for a Bastille Day fireworks show Thursday in the southern French city of Nice, killing at least 77 people and injuring 50 more in an apparent terror attack.

Regional president Christian Estrosi confirmed the death toll in a post on Twitter. Earlier, he had called the attack “the worst tragedy in the history of Nice” and asked residents to stay indoors.

French Interior Ministry spokesman Pierre-Henry Brandet said the truck driver had been “neutralized” and an investigation would confirm whether he had acted alone. The Paris prosecutor’s office announced that it was opening an anti-terrorism investigation into the attack.

The horror began at around 10:30 p.m. local time. A local government official told Fox News that the truck was driving full speed when it hit the crowd.

Damien Allemand, a journalist for the local Nice-Matin newspaper, said the fireworks display was over and people were getting up to leave when they heard a loud noise and screams.

“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.”

Allemand said people took shelter in a nearby restaurant, where he continued to hear people shouting for missing family members. He ventured out and saw bodies, blood and body parts all along the road.

“This evening, it was horror,” Allemand concluded.

Nice-Matin posted an image of the truck on Twitter, captioning it, “The truck that drove into the crowd.”

Another witness, Wassim Bouhlel, told the Associated Press that he saw the truck drive into the crowd, then witnessed the driver emerge with a gun and start shooting.

“There was carnage on the road,” Bouhlel said. “Bodies everywhere.”

Video footage showed men and women — one or two pushing strollers — racing to get away from the scenes. 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.

Estrosi confirmed to BFM TV that the driver fired on the crowd before being killed by police.

Nice’s public prosecutor told reporters early Friday that bodies of the victims were scattered for over a mile along the Promenade des Anglais, a major thoroughfare along the Mediterranean Sea.

Another witness, identified only as Chloe, told FranceTV Info, “We heard gunfire, a lot. A crowd came to us and told us to run … We went into a tapas bar and hid in the bathroom for half an hour.

“We went out and again we heard people running and saying ‘truck, truck,’ so we hid in the bathroom for an hour. The manager took us out by the back door and then I went home.”

There was no immediate claim of responsibility, but but France has recently seen a spate of dramatic assaults from by jihadist groups, including the Islamic State group (ISIS) which straddles Iraq and Syria.

A U.S. official told Fox News that the attack was in line with ISIS, which has become “increasingly brazen” in its attacks as it comes under increasing military pressure in Iraq and Syria.

Two sources, including a counterterror source who collects and monitors jihadist social media, told Fox News that accounts linked to ISIS were “celebratory” and their followers were told to use the hashtag “Nice”. A 2010 edition of Al Qaeda in the Arabian Peninsula’s propagada magazine “Inspire” also called on would-be terrorists to “use a pickup truck as a mowing machine … to mow down the enemies of Allah.”

President Barack Obama said he condemned “what appears to be a horrific terrorist attack” in the strongest terms.

“On this Bastille Day,” Obama said, “we are reminded of the extraordinary resilience and democratic values that have made France an inspiration to the entire world, and we know that the character of the French Republic will endure long after this devastating and tragic loss of life.”

Presumptive Republican presidential candidate Donald Trump announced late Thursday that he would postpone the announcement of his running mate, which had been scheduled to take place Friday morning.

France’s ambassador to the United States, Gerard Araud, characterized the events in Nice as a “terrorist attack.”

“Our democracies — France, the United States, our other partners, we are besieged, we face a terrible threat,” Araud said at a Bastille Day reception at the French Embassy in Washington late Thursday.

July 14 is a national holiday in France that commemorates the storming of the Bastille prison in Paris and the start of the French Revolution.

Fox News’ Catherine Herridge and The Associated Press contributed to this report.