The man who drove a truck down the seaside promenade in southern France, killing 84 and injuring hundreds of others who were celebrating Bastille Day, was a Tunisian national and French resident unknown to national intelligence services, a prosecutor said yesterday.

The 31-year-old man, identified as Mohamed Lahouaiej Bouhlel, was killed by police after driving 2 kilometres down Nice's main promenade starting shortly before 11pm (5am Malaysian time). Prosecutor Francois Molins said a driver's licence, bank card, portable phone and multiple weapons were found in the truck.

Warning that the victim count was still preliminary, Molins said that among the 84 dead, 10 were children or adolescents. He added that 202 people were injured, among whom 52 were in critical condition and 25 were on life support.

While Molins said that Lahouaiej Bouhlel was not known to have been radicalised, he said that the investigation would be devoted in part to determining ties with extremist organisations, saying the attack corresponded exactly to urgings broadcast by terrorist groups.

In Nice during the fireworks show for Bastille Day, France's national day, people rushed in panic away from the white delivery truck as it ploughed down the promenade, creating confusion and, according to eyewitness accounts to broadcasters, leaving a trail of bodies behind it.

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

Nice is the site of the third major attack on French soil in 19 months, after 149 people were killed in terrorist attacks last year. Hollande said that France faces a long struggle, "because we have an enemy that will continue to attack all the people and all the countries that hold liberty to be an essential value."

A delivery truck driver, Lahouaiej Bouhlel was married and had children. His estranged wife was taken into police custody yesterday morning and police searched two addresses where they seized documents and data, Molins said.

He was known to police and judicial services for threats and violence committed between 2010 and 2016. He was handed in March a suspended six-month sentence for a violent confrontation in January, but was "totally unknown" to national intelligence services and was not known to have been radicalised.

The truck was rented on July 11, and Lahouaiej Bouhlel arrived alone by bicycle to pick it up from where it was parked.

After ploughing down the promenade, Lahouaiej Bouhlel shot multiple times at three police officers, who followed the truck as it continued for another 300 metres, Molins said. After he was shot dead by police, two automatic pistols, a magazine, cartridges, two fake Kalashnikovs, and a non-functional grenade were found in the cab, Molins said.

An investigation was opened on the charges of murder, attempted murder of public authorities, and association with terrorist organisations.

France declared three days of national mourning to begin on Saturday, and flags in Nice flew at half-mast.

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

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.

Hollande spoke yesterday with several world leaders including US President Barack Obama, who vowed to stand united with US allies and destroy terrorist networks.

"These individuals and networks are an affront to all of our humanity," Obama said, pledging to stand with partners across the globe to "destroy this vile terrorist organisation."

After holding a crisis meeting and defence council, Hollande said France would continue to deploy exceptional measures. Approximately 10,000 soldiers were deployed in France following attacks on the office of satirical magazine Charlie Hebdo in January 2015.

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

"Germany stands side by side with France in the fight against terrorism," German Chancellor Angela Merkel said.

"And I am very confident that we will win this fight, despite the difficulties," Merkel said. She added that France's national holiday had been hit by terror, which is "a day of pride but has now become a day of great sadness."

- dpa