France has declared three days of national mourning from Saturday after at least 84 people were mown down by a truck while watching a Bastille Day fireworks display, the prime minister said.
Manuel Valls also told reporters that the government wants to extend the state of emergency which has been in force since the November 13 Paris attacks, until October.
Flags will be flown at half-mast from Friday, and a law extending increased powers for the police will be put before parliament next week, he said.
"Times have changed, and France is going to have to live with terrorism, and we must face this together and show our collective sang-froid," he said.
"France is a great country and a great democracy and we will not allow ourselves to be destabilised," he added.
"We want to bring the French nation together. The only dignified response (to the attack) is for France to stick with the spirit of July 14, a France that is united around its values," he said.
Bastille Day is the country's national holiday and marks the start of the French Revolution in 1789.
It is celebrated with military parades and fireworks displays across the country, and is the traditional start of the holiday session.