A large truck plowed through revellers gathered for Bastille Day fireworks in Nice, leaving at least 84 people dead and sending others fleeing into the sea as it bore down for two kilometres along the French Riviera city's famed waterfront promenade.

The driver was killed by police and no one immediately claimed responsibility for the Thursday night attack on France's national holiday. It has rocked a nation still reeling from two attacks in Paris last year that killed a total of 147 people.

Of the 202 wounded, 25 are on life-support and 52 are in critical condition, according to Paris prosecutor François Molins. Ten children were among the dead. 

French President François Hollande earlier condemned the "unspeakable act." 

"Why Nice? Because it's a town famous across the world, a beautiful town, one of the most beautiful on the planet," Hollande said.

'He was trying to hit everyone in his way.' - Sanchia Lambert, eyewitness 

"Why the 14th of July? Because it's the celebration of Bastille Day — freedom."

The truck plowed into the crowd over two kilometres and broadcast footage showed a scene of horror up and down the promenade, with broken bodies splayed on the asphalt, some piled near one another, others bleeding onto the roadway or twisted into unnatural shapes.

nice-truck-path

Some tried to escape by jumping into the water. 

The survivors will suffer life-long psychological trauma from what they saw, Hollande said.  

Police named the driver as 31-year-old Mohamed Lahouaiej Bouhlel, a Tunisian-born Frenchman and petty criminal.

Several neighbours said police raided a 12th-floor apartment linked to Bouhlel earlier Friday, but that he had not lived there in three years. The apartment was occupied by his estranged wife, who has been arrested, authorities said later.

'Attacks aren't prepared alone.' - Christian Estrosi, regional president

Bouhlel was not on the watch list of French intelligence services, but was known to the police in connection with common crimes such as theft and violence. Investigators are looking for any ties to extremist groups. 

An eyewitness to the attack described the driver as "a young guy, very focused on what he was doing." 

"He didn't looking angry," John Lambert told CBC News. "He wasn't screaming or shouting. He was just … focusing on the job." 

His wife, Sanchia Lambert, was filming the celebration when the attack began. 

"I couldn't see his face," she said of the driver. 

"I saw his arms and the way he was zigzagging and it was deliberate … he was trying to hit everyone in his way."

Two Americans are among the dead, identified by relatives as 51-year-old Sean Copeland and his 11-year-old son Brodie of Lakeway, Texas. 

Family friend Jess Davis released a statement Friday on behalf of the Copeland family, saying they are "heartbroken and in shock."

Davis said the Copelands were on a European vacation that began in Spain. 

No Canadians have been reported among the casualties, according to a tweet from Canada's ambassador to France, Lawrence Cannon. 

However, a group of students from MacEwan University in Edmonton attending a program in Nice is reporting one member of their party appears to be missing

FRANCE NICE TRUCK TERROR ATTACKS AFTERMATH

A mourner places flowers in front of the memorial set on the Promenade des Anglais where a truck crashed into the crowd during the Bastille Day celebrations in Nice, France. (Ian Langsdon/EPA)

3 days of mourning

The regional president, Christian Estrosi, told BFM TV that "the driver fired on the crowd, according to the police who killed him." 

Flags were lowered to half-mast in Nice and Paris, and Hollande extended the state of emergency imposed after the November bloodshed another three months. The government declared three days of national mourning to begin Saturday.

People flee after a truck plows into crowd in Nice, France1:21

Officials said it was not immediately clear whether the driver had accomplices. 

Estrosi said some of the city's 1,200 security cameras had pinpointed the moment the attacker boarded the truck, far from the seaside "in the hills of Nice," and could follow his path to the promenade. Estrosi called for the investigation to focus on any accomplices.

"Attacks aren't prepared alone. Attacks are prepared with accomplices," Estrosi said. "There is a chain of complicity. I expect it to be unveiled, discovered and kept up to date."

France has lived with soldiers in the streets since the November attacks, and much of the country was under intense security during the month-long European football championships, which ended July 10 without incident.

'He was trying to hit as many people as possible': Nice eyewitness7:24

Video footage showed men and women — one or two pushing strollers — racing to get away from the scene of Thursday's attack. Photos showed a truck with around 20 bullet holes.

Writing online, Nice Matin journalist Damien Allemand, who was at the waterside, said the fireworks display had finished and the crowd had been getting ready 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."

On video footage, one person could be heard yelling, "Help my mother, please!" A girl's pink bicycle was overturned by the side of the road.

10,000 soldiers on patrol

Hollande announced a series of measures to bolster security. Besides continuing the state of emergency and the Sentinel operation with 10,000 soldiers on patrol, he said he was calling up "operational reserves," those who have served in the past and will be brought in to help police, particularly at French borders.

He reiterated that France is also bolstering its presence in Iraq and Syria, where he said earlier that military advisers would be on the ground to help Iraqis take back the Islamic State in Iraq and Syria (ISIS) stronghold of Mosul.

U.S. President Barack Obama condemned what he said "appears to be a horrific terrorist attack."

European Council president Donald Tusk said it was 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.

FRANCE NICE TRUCK ATTACK

Police researchers inspect the scene where the truck crashed into the crowd. (Alberto Estevez/EPA)

France has long known it is a top target for ISIS. In September 2014, then spokesman Abu Mohammed al-Adnani referred to "the filthy French" in a statement telling Muslims within the country to attack them in any way they could, including "crush them with your car."

France has been not only a prime target for ISIS, but has seen more young Muslims head to join extremists than any other European nation. And Nice has been at the centre of the movement, home to one of the most prolific creators of jihadi recruiting videos for the French-speaking world, a former petty drug dealer named Omar Omsen who is now fighting in Syria.

ISIS carried out the November 2015 attacks, sending a group of largely French-speaking men to strike at France's national stadium, bars and cafes and the Bataclan concert venue. In March, the same ISIS cell struck in Belgium.

EUROPE-ATTACKS/NICE

A visibly shaken man sits near a French flag along the beachfront the day after the truck attack. (Eric Gaillard/Reuters)

With files from CBC News and Reuters