At least 77 people have died in the French city of Nice after an attacker plowed a truck, which was "loaded with" grenades and other arms, into a large crowd during Bastille Day celebrations Thursday night, officials said.

The driver pointed his tractor-trailer into the crowd and accelerated, mowing bodies over. The driver was shot to death, authorities said.

The incident left an "apocalyptic" scene, according to eyewitnesses, on the famed Promenade des Anglais, put the city on lockdown and drew swift condemnation from world leaders.

"On behalf of the American people, I condemn in the strongest terms what appears to be a horrific terrorist attack in Nice, France, which killed and wounded dozens of innocent citizens," President Obama said in a statement. "Our thoughts and prayers are with the families and other loved ones of those killed, and we wish a full recovery for the many wounded."

The American consulate in Marseille said it was trying to "determine if any U.S. citizens were injured in the event."

The exact nature of the attack was unclear, but the Paris anti-terrorism prosecutor's office was put in charge of the investigation.

Christian Estrosi, president of the Nice region, told French TV that the motorist "fired on the crowd, according to the police who killed him" and said that "the truck was loaded with arms, loaded with grenades."

The truck was driven by someone who appeared to have "completely premeditated behavior," Estrosi added.

According to Eric Ciotti, who represents Nice in the French parliament, 77 people were killed and another 65 injured, including 15 critically, Ciotti said.

Jimmy Ghazal, 39, a Lebanese man visiting Nice with his family, told ABC News that after the fireworks concluded, he heard people screaming and heard what sounded like gunshots. He saw people run to shelter.

Ghazal said he saw a big truck drive "through all the security.”

“The kids thought it was part of the fireworks," he told ABC News. "We just told them it was part of the fireworks.”

Video showed crowds of screaming people running through the streets in panic.

Earlier Thursday, French President Francois Hollande had said France’s state of emergency -- which has been in place since the Paris terror attacks in November -- would be lifted later this month.