At least 84 people were killed and another 100 injured in the southern French city of Nice after an attacker drove a lorry into a crowd celebrating the Bastille Day national holiday, authorities said.

The attacker behind the wheel on Thursday drove at high speed along the famed Promenade des Anglais seafront, hitting the mass of spectators who had been watching a fireworks display.

Police later shot and killed the driver, officials said.

France's Interior Minister Bernard Cazeneuve said 80 people were killed, including children, and 18 were in critical condition.

Identity papers belonging to a 31-year-old French-Tunisian were found inside the truck, a police source told the AFP news agency on Friday.

"The identification of the truck driver is still under way," said the source. The identity papers indicate the man is a resident of Nice.

The attack struck just as the fireworks display - which was attended by thousands of people - was ending, at around 11:00pm local time (21:00GMT).

Eric Ciottio, the ranking politician of the Alpes-Maritime department that includes Nice, said the lorry rammed into the crowd over a distance of two kilometres.

Ciotti said on BFM TV that police killed the driver "apparently after an exchange of gunfire". He said the truck was loaded with weapons and grenades.

In a live television address on the early hours of Friday, French President Francois Hollande said the attack was clearly a "terrorist" act.

"There's no denying the terrorist nature of this attack of yet again the most extreme form of violence," he said.

Hollande said he would extend France's state of emergency, which has been in place since the deadly attacks in Paris last November, for another three months from July 26.

"Nothing will make us yield in our will to fight terrorism. We will further strengthen our actions in Iraq and in Syria. We will continue striking those who attack us on our own soil," he said, in reference to the Islamic State in Iraq and the Levant (ISIL, also known as ISIS) armed group.

Residents of Nice, a Mediterranean city close to the Italian border, were advised to stay indoors.

There was no immediate claim of responsibility for the attack. But Nada Bakos, former CIA analyst, told Al Jazeera that it "looks very similar" to some of the attacks carried out by al-Qaeda and ISIL.

She said such groups "had a playbook that essentially foretold this type of attacks".

"They were inciting their followers to produce this type of violence."

'I saw bodies flying'

Writing online, Nice Matin journalist Damien Allemand, who was at the waterside, said the fireworks display had finished and the crowd was about to leave when they heard a noise and cries.

"A fraction of a second later, an enormous white truck came along at a crazy speed, turning the wheel to mow down the maximum number of people," he said.

"I saw bodies flying like bowling pins along its route. Heard noises, cries that I will never forget."

Allemand, along with others, took shelter in a nearby restaurant. He continued to hear people shouting for missing family members. He ventured out and saw bodies, blood and body parts all along the road.

"This evening, it was horror," Allemand concluded.

The Paris prosecutor's office opened an investigation for "murder, attempted murder in an organised group linked to a terrorist enterprise". The investigation is being handled by France's intelligence agency and judicial police.

An eyewitness told the Associated Press news agency that he saw the driver emerge from the lorry shooting, after ramming into the crowd.

"There was carnage on the road," Wassim Bouhlel, a Nice native, told AP. "Bodies everywhere."

Al Jazeera reporter David Coady was also at the scene.

"I was enjoying the Bastille Day fireworks just like thousands of other people at the promenade in Nice," he said.

"I was just walking back after the fireworks had finished. I looked towards the truck and I thought it was a bit odd, because the roads were shut down and there were hundreds, perhaps thousands, of people walking along.

"But then, from the direction of the truck, I started hearing screaming and then people started running. And so I joined all those people in running away from there."

Obama: 'A horrific terrorist attack'

US President Barack Obama released a statement condemning "what appears to be a horrific terrorist attack".

European Council president Donald Tusk said it is a "tragic paradox" that the victims of the attack in Nice were celebrating "liberty, equality and fraternity" - France's motto - on the country's national day.

Tusk tweeted a photograph of himself and other European and Asian leaders standing in tribute to the Nice victims at the Asia-Europe meeting in Mongolia.

Almost exactly eight months ago, attackers linked to ISIL killed 130 people in Paris. November 13 saw some of the bloodiest in a number of attacks in France and Belgium over the past two years.

On Sunday, France had breathed a sigh of relief as the month-long Euro 2016 football tournament ended without a feared attack.

Hollande, who was in the south of France at the time of the attack but raced back to Paris to the national crisis centre, had hours earlier said that a state of emergency put in place after November's Paris attacks would not be extended when it expires on July 26.

Agencies