In a pre-dawn address to the nation, President Francois Hollande called 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 has been extended.
Hollande noted 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.
Interior Minister Bernard Cazeneuve said 80 people died and 18 were in a critical condition, with many more also wounded.
Counter-terrorist investigators were seeking to identify the driver, who a local government official said opened fire before police shot him dead. The official said weapons and grenades were found inside the 25-tonne, unmarked articulated truck.
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.
The truck careered for hundreds of metres along the famed Promenade des Anglais seafront, slamming into spectators watching the fireworks, listening to an orchestra or strolling above the beach toward the grand, century-old Hotel Negresco.
Police told residents of the city, 30 km from the Italian border, to stay indoors as they conducted further operations, although there was no sign of any other attack.
Hours earlier, Hollande, who raced back to Paris from the south of France after the attack, had said the state of emergency would end in two weeks. He has now extended it by three months, calling up former troops and gendarmes.
US President Barack Obama said in a statement: "On behalf of the American people, I condemn in the strongest terms what appears to be a horrific terrorist attack in Nice, France, which killed and wounded dozens of innocent civilians."