Paris, France | AFP: France had declared three days of national mourning from Saturday after about 90 people were mown down by a truck while they were 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.
Meanwhile, another AFP story filed from London said: British Prime Minister Theresa May called an emergency national security meeting Friday after the Bastille Day attack in the French city of Nice, in which at least 84 people were killed.
May, who was due later Friday in Scotland, said she had asked her deputy national security adviser to chair the COBRA emergencies committee, which groups top ministers and security chiefs.
The meeting was aimed at reviewing "what we know and what we can do to help", she said in a statement, expressing shock over the "horrifying attack".
A government spokeswoman said there had been a "small number of injured" British nationals.
May took office on Wednesday after David Cameron stepped down as premier after Britain’s shock June 23 vote to leave the European Union.
"We must work with France and our partners around the world to stand up for our values and for our freedom," she said.
"If, as we fear, this was a terrorist attack then we must redouble our efforts to defeat these brutal murderers who want to destroy our way of life," she added.
Her assurances of cross-Channel cooperation echoed remarks by Foreign Secretary Boris Johnson, who campaigned for Britain to end its 43-year period in the European fold.
Voicing shock at the "appalling" assault, Johnson, who was the star guest at the French ambassador to London’s Bastille Day party Thursday, told the BBC that terror "represents a continuing threat to us in the whole of Europe and we must meet it together".
Meanwhile, London Mayor Sadiq Khan said he was reassessing security levels in the capital.
"We will be reviewing our own safety measures in light of this attack," he said.
Expressing solidarity with the French, he added: "Londoners today stand united with Nice and all of France in our grief.
"They will not win. Not in France, not in London, not anywhere."
While Paris and Brussels have both suffered devastating jihadist attacks in the past year, Britain has been spared a major assault since the July 2005 bombings in London’s public transport system.
After the November attacks in Paris, in which 130 people were killed, English football fans attending an England-France match memorably sang the French national anthem to show solidarity with their neighbours.