France declares three days of national mourning

France on Friday announced three days of national mourning after the terror attack in Nice city where a truck driver mowed down at least 84 Bastille Day revellers.

French Prime Minister Manuel Valls said the national mourning would be observed from Saturday, July 16-18. Flags on government buildings will fly at half-mast.

"We would like to tell the French people that we will never give in," Valls said outside the Élysee Palace in Paris.

"Terrorism, as we've been saying for so long, is a threat that weighs heavily on France, and will still weigh [heavily] for a long time," he said, adding that France will need to learn to live with terrorist attacks.

"France, once again, has been hit in its soul, on the 14th of July, our National Day. They wanted to attack the unity of the French nation."

France will remain "united and joined around its values", Valls said.

The Prime Minister also said the government has activated a number of crisis teams, including a high-level group set up to deal with terrorism and a hospital and government network to aid victims and their families.

Valls said a bill to extend the country's state of emergency by three months will be submitted in parliament on Tuesday. Parliament will study the bill for two days, he told reporters.

The department of Alpes-Maritimes, which includes Nice, will remain on high alert.

Valls reiterated that a state of emergency that was due to be lifted this month would remain in place for three more months, along with a plan for heightened military patrols across France.

Hundreds were out on the streets, celebrating Bastille Day, France's answer to the US's July 4th Independence Day, with fireworks and a beachfront concert in Nice when the carnage took place.