The deaths rocked a nation still dealing with recent attacks: in November, in Paris, when 130 were killed, and in January, 2015, when 17 were killed.
“All of France is under the threat of Islamic terrorists,” Mr Hollande said yesterday.
The Lenval Children’s Hospital, in Nice, has treated 50 youngsters aged under 18 who were injured in the attack, including two who died during or after surgery.
Stephanie Simpson, the hospital communications director, said “some are still life and death.”
The hospital, equipped with one of France’s largest paediatric emergency units, had to free up rooms for the attack victims.
Footage showed a scene of horror up and down the promenade, with bodies piled near one another and others bleeding on the road.
German tourist, Richard Gutjahr, heard angry shouts and saw a big, white truck rolling slowly down a road blocked off as a party zone. He filmed a key moment. Gutjahr’s footage, taken from a first-floor hotel balcony, shows the truck chased by police officers. An unidentified motorcyclist pulls alongside the truck, leaps off his scooter and clings to the door of the truck to stop the attacker. The motorcycle is partly crushed under the truck’s tires as the motorcyclist, possibly a police officer, clings on. Gutjahr saw two other officers, on foot, take aim and fire shots at the truck’s windscreen.
Gutjahr said: “I thought it could either be a drunk driver or a terror attack, until that incredibly brave man jumped on the truck.”
Wassim Bouhlel, a Nice native, said he saw the lorry drive into the crowd. “There was carnage on the road,” he said. “Bodies everywhere.” He said the driver emerged with a gun and started shooting.
Crowds dressed in summer clothes ran for their lives down the Promenade des Anglais, the famous seaside boulevard named after the English aristocrats who proposed its construction in the 19th century.
“France was struck on the day of its national holiday, July 14, the symbol of liberty,” Mr Hollande said, denouncing “this monstrosity” — a truck bearing down on citizens “with the intention of killing, smashing and massacring ... an absolute violence”.
Mr Hollande said it was not clear whether the driver had accomplices. The Paris prosecutor’s office opened an investigation for “murder and attempted murder in an organised group linked to a terrorist enterprise”.
France has lived with soldiers in the streets since the November attacks, security intensified during the month-long European football championships, which ended on July 10 without incident.
The regional president, Christian Estrosi, told BFM TV that “the driver fired on the crowd, according to the police who killed him”. The truck had 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 begun leaving 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 shouting: “Help my mother, please!” A pink girl’s bicycle was overturned by the side of the road.
Mr 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.