"We are facing a war that terrorism is inflicting on us," Valls said after a meeting in the Elysee Palace, adding "the only response is that of a united France."
French flags will be flown at half mast on all public buildings in the country, Valls added.
The death toll continues to rise from an attack in which a truck drove through a crowd, ramming into people celebrating the French national day in Nice, Interior Minister Bernard Cazeneuve said on Friday.
At least 84 people have been killed and 18 remain in "critical condition," said the minister. French President Francois Hollande said the death toll included "several children."
According to local media, at least 50 children have been hospitalized.
French President François Hollande announced he would extend the current state of emergency in the country for another three months, despite saying Thursday it would not be extended past July 26.