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 80 people and injuring 50 more in an apparent terror attack.

There was no immediate claim of responsibility, but two sources, including a U.S. counterterrorism 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.

French Interior Ministry spokesman Pierre-Henry Brandet said the truck driver had been "neutralized", or killed by police, 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.

Witnesses told France's BFM TV that the driver fired shots into the fleeing crowd on the Promenade des Anglais, a major boulevard along the Mediterranean Sea.

Warning: Extremely graphic video.

French President Francois Hollande said that children were among the dead, and said his country was "under the threat of Islamic terrorism. We have to demonstrate absolute vigilance and show determination that is unfailing."

Hollande also announced that he would extend France's state of emergency by another three months, until Oct. 26. France has been on its highest state of alert since ISIS terrorists killed 130 people in Paris this past Nov. 13.

Nice Mayor Christian Estrosi asked residents to stay where they were immediately following the attack. He called the attack "the worst tragedy in the history of Nice."

The attack happened at around 10:30 p.m. following a fireworks display. The Nice-Matin newspaper quoted one of its reporters as saying the truck driver "mounted onto the Prom and he drove on everyone." The reporter added, "people are running, it's panic."

A photo taken by the reporter showed around a dozen people lying in the street, with some being tended to by emergency personnel.

The paper posted an image of the truck on Twitter, captioning it, "The truck that drove into the crowd."

Twitter user harp_detectives filmed people running down the street in panic away from the carnage. Glasses can be heard breaking as people scramble to get out of the cafe.

President Obama's national security spokesman said the president will be kept up to date on the events unfolding.

Donald Trump tweeted his reaction to the suspected attack, calling it "horrific" and asking "when will we learn?" He also announced that he would be postponing the planned announcement of his vice presidential pick.

Stay tuned to Fox News Channel and check back for updates.