At least 84 people were killed and dozens injured when a truck plowed into a crowd celebrating Bastille Day along the waterfront promenade in the southern French city of Nice, hitting a country still on high alert from a string of terrorist attacks.

Nearly 20 people remained in critical condition on Friday morning. The truck drove 2 kilometres along Nice's crowded main promenade just as a fireworks show for France's national day was ending, sparking panic as people tried to escape.

The driver was shot and killed by police, a spokesman for the Interior Ministry said. An investigation into the incident was handed over to the anti-terrorism section of the Paris prosecutor's office.

"It is clear that we must do everything in order to fight against the scourge of terrorism," French President Francois Hollande said in the early hours of Friday morning. "It is all of France that is under the threat of Islamist terrorism."

Pledging to continue to fight against terrorism, Hollande said France would reinforce military actions in Iraq and Syria. France has contributed to airstrikes in both countries, targeting the Islamic State group.

French news agency AFP said the driver had been formally identified as a 31-year-old French-Tunisian, and officials continued searching for possible accomplices. Broadcaster BFMTV reported that a bank card and a telephone had been found in the truck, along with weapons.

Officials declined to talk about the identity of the man, saying that prosecutors would give relevant information in due course.

A local politician and president of the region, Christian Estrosi, told broadcasters that surveillance videos had captured images of the driver. Nice is among the cities in France with the highest number of security surveillance cameras, French media reported.

Amateur videos of the attack showed people running along the streets of the city. Several witnesses told broadcasters that they saw the truck leave a trail of victims, and multiple people posted messages on social media seeking information about their friends and families.

Switzerland's Foreign Department said a Swiss woman was among the dead. The Department is trying to clarify whether other Swiss nationals were killed in the attack.

Hollande said a state of emergency - imposed after a string of terrorist attacks at a concert hall, national stadium and cafes in Paris on November 13 - would be extended by six months beyond its current expiration date of July 26.

He also travelled back to Paris from Avignon to hold a crisis meeting with Prime Minister Manuel Valls, who later declared three days of national mourning beginning Saturday.

On Friday morning, the president held a defence council meeting to assess security measures in place and those that would be taken in response to the attack. Approximately 10,000 soldiers were deployed in France following attacks on the office of satirical magazine Charlie Hebdo in January 2015.

Hollande and Valls were scheduled to travel to Nice after the defence council. Along the promenade of Nice, the French tricolor flew at half-mast under a clear blue sky.

The vehicle that was used to plow through the crowd was still near the promenade on Friday morning. Television images showed a bullet-pocked white heavy delivery truck blocked with police tape.

Nice had already been under high security, along with the rest of France, with extra security measures in place for the Bastille Day celebrations and local and national police deployed throughout the country.

France hosted the Euro 2016 football championships through July 10, attended by hundreds of thousands of visitors under tight security.

US President Barack Obama condemned "what appears to be a horrific terrorist attack" and offered France "any assistance that they may need."

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