The driver was shot dead after barrelling the truck two kilometres (1.3 miles) through the festive crowd on the palm-lined Promenade des Anglais, sending hundreds fleeing in terror and leaving the area strewn with bodies.
Authorities said they found identity papers belonging to a 31-year-old French-Tunisian citizen in the truck, as well as “guns” and “larger weapons.”
The attack was of an “undeniable terrorist nature,” a sombre Hollande said in a televised national address, confirming that “several children” were among the dead as families came together to celebrate France’s national day.
The bloodshed came on Bastille Day, a celebration of everything France holds dear, its secular republic and the values of “Liberte, Egalite, Fraternite” (Freedom, Equality, Fraternity).
The attacker struck after a day of military pomp and ceremony in Paris — where armed forces, tanks and fighter jets swooped down the Champs Elysees avenue — and spectacular firework displays.
“France was struck on its national day … the symbol of freedom,” said Hollande.
A photograph showed the front of the truck riddled with bullet holes and badly damaged, with burst tyres, while a lone doll lay abandoned on the promenade where families celebrated the holiday just hours earlier.
Robert Holloway, an AFP reporter who witnessed the white truck driving at speed onto the seaside road, described scenes of “absolute chaos”.
“We saw people hit and bits of debris flying around. I had to protect my face from flying debris,” he said.
– ‘Horrific terrorist attack’ –
The attack is the third major strike against France in less than 18 months and prosecutors said anti-terrorist investigators would handle the probe.
It comes eight months after Islamic State attacks on Paris nightspots left 130 people dead, dealing a hard blow to tourism in one of the world’s top destinations.
US President Barack Obama condemned “what appears to be a horrific terrorist attack”, although no group had yet claimed responsibility.
Hollande announced he would extend France’s state of emergency for three months in the wake of this latest attack and “step up” the government’s action against jihadists in Syria and Iraq.
He also called up army reservists to bolster the country’s security services that are stretched to the limit.
France has been under a state of emergency ever since the November 13 Paris carnage, which came after 17 were killed in another attack in January at various sites including the offices of the Charlie Hebdo magazine and a Jewish supermarket.
The Islamic State group has repeatedly singled out France as a prime target for its military actions against the group in Iraq and Syria, and hundreds of jihadists have left France to go and fight in its ranks.
– ‘People were tripping over’ –
The Mediterranean city of Nice, with its pebble beaches and clear blue water, has been a magnet for sun-seekers and the jet-set since the 19th century.
A concert by popstar Rihanna due Friday was cancelled in the wake of the attack, as well as the Nice Jazz festival.
An Australian caught up in the chaos, Emily Watkins, told the Australian Broadcasting Corporation that while she saw the truck she did not realise what had happened.
“There was a lot of screams coming from ahead of us where the truck was,” she said.
“People were tripping over and trying to get into hotel lobbies and restaurants or car parks or anywhere they could to get away from the street.”
The Promenade des Anglais was sealed off, crawling with police and ambulances as authorities from the local Alpes-Maritimes prefecture urged residents to stay indoors.
Over the past week, France had been breathing a sigh of relief after successfully hosting the month-long Euro 2016 football championship, which passed off without incident despite fears of attacks.
The tournament brought an all-too-brief burst of joy to a gloomy France, bogged down after the two attacks in 2015, violent anti-government protests, strikes and floods.
The attack sent shockwaves across the globe with China offering “condolences” and US Republican presidential hopeful Donald Trump postponing the announcement of his pick for running mate because of the attack.
European Council President Donald Tusk said: “It’s a sad day for France, for Europe, for all of us.”
“The subjects of the attack were people celebrating liberty, equality and fraternity.”