At least 10 children and teenagers were among the dead, mown down as Mohamed Lahouaiej-Bouhlel ploughed his lorry through the festive crowd of thousands watching a fireworks display for France's national day.
France's anti-terrorism prosecutor Francois Molins said the 31-year-old Lahouaiej-Bouhlel was "completely unknown" to the intelligence services but that the attack was "exactly in line with" calls from jihadist groups to kill.
The attack left a scene of carnage on Nice's famous Promenade des Anglais, with mangled bodies strewn over the palm tree-lined walkway.
AFP reporter Robert Holloway witnessed the white truck driving at speed into the crowd, causing "absolute chaos".
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"It was hurtling towards us and we had just enough time to yell at each other 'get out of the way!'," he said.
One witness said a motorcyclist tried to stop the rampage by drawing level with the truck and attempting to open the door of its cab before he fell and ended up under its wheels.
The massacre again prompted questions as to why France is such a persistent target for such attacks and what can be done to prevent such a unsophisticated assault.
Hollande's political opponents were already asking questions as to how such a tragedy could strike France again, with French presidential contender Alain Juppe saying it could have been avoided if "all measures" had been taken.
Hollande said many foreigners were among the dead and injured in what he described as a "terrorist attack."
Two US citizens, a Ukrainian and a Russian woman were killed, their countries said.
Hollande said France would observe three days of national mourning tomorrow, Paris city hall said the Eiffel Tower will be lit in France's national colours today in tribute to the victims.
A shellshocked France found itself in mourning again after attacks on Charlie Hebdo magazine in January 2015 and the shootings and suicide bombings in Paris last November left a total of 147 dead.
The country has been in a state of emergency since November with heightened security, but the Nice carnage showed how vulnerable it remains to an attack by what appeared to be a determined individual.