A 31-year-old French-Tunisian killed 84 people when he drove a truck through crowds celebrating Bastille Day in Nice.
French president Francois Hollande and prime minister Manuel Valls have been visiting the city before three days of national mourning begin on Saturday.
Speaking after a visit to a hospital in Nice this afternoon, Mr Hollande said that some 50 people were "between life and death".
The French government has extended the country's nine-month-old state of emergency and vowed to deploy thousands of police reservists on the streets after the massacre of pedestrians leaving a fireworks display for France's national independence day.
Video shot by terrified civilians shows crowds fleeing in panic on the famous waterfront Promenade des Anglais, leaping off the elevated pavement on to the beach below before police finally surrounded the stationary truck and shot dead the driver.
The deaths rocked a nation still dealing with the aftermath of attacks in November in Paris that killed 130 and in January 2015 that killed 17.
"All of France is under the threat of Islamic terrorists," Mr Hollande said on national television early on Friday.
The children's hospital in Nice said it has treated about 50 youngsters injured in the attack, including two who died during or after surgery.
Stephanie Simpson, the communications director for the Lenval foundation hospital, said that injuries included fractures and head injuries and that the victims were aged 18 or under.
She said: "Some are still life and death."
The hospital, equipped with one of France's largest paediatric emergency units, also called the families of children it was treating before the attack to ask them to pick up their children to free up rooms for the attack victims.
Footage showed a scene of horror up and down the promenade, with bodies on the floor, some piled near one another and others bleeding on the road.
Some tried to escape into the water, a politician for the region that includes Nice said, giving new details of the horrifying last minutes of the attack.
"A person jumped on to the truck to try to stop it," Eric Ciotti told Europe 1 radio. "It's at that moment that the police were able to neutralise this terrorist. I won't forget the look of this policewoman who intercepted the killer."
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.
Interior Minister Bernard Cazeneuve, who travelled to the scene, said police were trying to identify the driver.
Two French police officials said identity papers found alongside the attacker belonged to a 31-year-old Frenchman of Tunisian descent with previous minor convictions but no known link to extremist groups.
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 immediately 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".
Mr Cazeneuve said "we are in a war with terrorists who want to strike us at any price and in a very violent way".
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 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".
Video footage showed men and women - one or two pushing prams - racing to get away from the scene. The truck had 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 got up 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 of the attack, 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.
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 stronghold of Mosul.
US 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.
