Interior Minister Bernard Cazeneuve said 80 people died and 18 were in a critical condition. Many more were also wounded in the attack along the famed seafront Promenade des Anglais as the fireworks ended just after 10:30 p.m. (2030 GMT).
The driver also opened fire before police shot him dead.
In a pre-dawn address to the nation, President Francois Hollande said he was calling up military and police reservists to relieve forces worn out by an eight-month state of emergency begun after the Islamic State militant group killed 130 people in Paris. The state of emergency was extended by three months.
"France is filled with sadness by this new tragedy," Hollande said, noting several children were among the dead in what he said he had no doubt was an act of terrorism.
He called the carnage, which came as France celebrated the anniversary of the 1789 revolutionary storming of the Bastille, an attack on liberty by fanatics who despised human rights.
France would, nonetheless, continue military operations in Syria and Iraq.
Counter-terrorist investigators were seeking to identify the driver. A local government official said weapons and grenades were found inside the 25-tonne, unmarked articulated truck.
Officials said hundreds were hurt as the driver wove along the seafront, knocking them down "like skittles".
The attack, which came eight months and a day after Islamic State gunmen and suicide bombers struck the French capital on a festive Friday evening, seemed so far to be the work of a lone assailant. Newspaper Nice-Matin quoted unidentified sources as saying the driver was a 31-year-old local of Tunisian origin.
Police were working to establish what accomplices he may have had in a city with a reputation for Islamist activism.
There had been no claim of responsibility made almost six hours after the attack.
"A SCENE OF HORROR"
The truck careered for hundreds of meters along the front facing the Baie des Anges (Bay of Angels), slamming into families and friends listening to an orchestra or strolling above the beach toward the grand, century-old Hotel Negresco.
"It's a scene of horror," member of parliament Eric Ciotti told France Info radio, saying the truck "mowed down several hundred people." Jacques, who runs Le Queenie restaurant on the seafront, told the station: "People went down like ninepins."
Bystander Franck Sidoli, who was visibly shocked, said: "I saw people go down."
"Then the truck stopped, we were just five meters away. A woman was there, she lost her son. Her son was on the ground, bleeding," he told Reuters at the scene.
Nice-Matin posted photographs of the truck, its windshield starred by a score of bullets and its radiator grille destroyed.
Major events in France have been guarded by troops and armed police since the Islamic State attacks last year, but it appeared to have taken many minutes to halt the progress of the truck as it tore along pavements and a pedestrian zone.
Police told residents of the city, 30 km (20 miles) from the Italian border, to stay indoors as they conducted further operations, although there was no sign of any other attack.