One of the dozens of injured on the ground after a truck ploughed into a Bastille Day crowd in Nice, France. Reuters/Eric Gaillard

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At least 30 people were killed and 100 injured in the French Riviera city of Nice late on Thursday when a truck ploughed into crowds watching a fireworks display on France's Bastille Day national holiday in a criminal attack, a local official said.

The driver, who drove at high speed for over 100 meters (yards) along the famed Promenade des Anglais seafront before hitting the mass of spectators, was shot dead, sub-prefect Sebastien Humbert told France Infos radio.

Humbert described it as a clear criminal attack, although the driver was not yet identified. Residents of the Mediterranean city close to the Italian border were advised to stay indoors. There was no sign of any other attack.

Police in Nice, France, after a truck drove into a crowd on Bastille Day. Reuters/Eric Gaillard

Almost exactly eight months ago Islamic State militants killed 130 people in Paris. On Sunday, France had breathed a sigh of relief as the month-long Euro 2016 soccer tournament ended without a feared attack.

"Dear Nicois," local mayor Christian Estrosi tweeted, "The driver of a truck appears to have killed dozens of people. Stay at home for the time being. More news to follow."

Regional newspaper Nice Matin quoted its reporter at the scene saying there were many injured people and blood on the street. It published a photograph of a damaged, long-distance delivery truck, which it said was riddled with bullets and images of emergency services treating the injured.

Damien Allemand, the paper's correspondent, was quoted as saying: "People are running. It's panic. He rode up onto the Prom and piled into the crowd ... There are people covered in blood. There must be many injured."

French police on the street in Nice after Bastille Day attack. Reuters/Eric Gaillard

Social media carried images of people lying apparently lifeless in pools of blood.

U.S. government agencies have received constant reports of Islamic State threats to attack France and those threats are regarded as current, a U.S. security official said. However, two U.S. officials said they had no information at this point about whether militants were involved in the Nice incident.

CNN said it has spoken to a witness, identified as an American pilot, who saw the truck ramming the crowd. The witness said the driver mowed people down, accelerating as he hit them. The witness said there was only one person in the truck.

Local mayor Estrosi has warned in the past of the risk of Islamist attacks in the region, following Islamic State bloodshed in Paris and Brussels over the past 18 months.

A street in Nice, France, after a Bastille Day incident killed dozens of people. Reuters/Eric Gaillard

French President Francois Hollande, who was in the south of France at the time, had hours earlier said a state of emergency put in place after the Paris attacks in November would not be extended when it was due to expire on July 26.

"We can't extend the state of emergency indefinitely, it would make no sense. That would mean we're no longer a republic with the rule of law applied in all circumstances," Hollande told journalists in a traditional Bastille Day interview.