France has called up 12,000 police reservists to help boost security after Thursday's attack in Nice in which more than 80 people were killed.
Interior Minister Bernard Cazeneuve also appealed to "all willing French patriots" to sign up as reservists, to help protect the country's borders.
Mohamed Lahouaiej-Bouhlel drove a lorry along the sea front through crowds before police shot him dead.
So-called Islamic State claimed one of its followers carried out the attack.
A news agency linked to the group, Amaq Agency, said: "He did the attack in response to calls to target the citizens of the coalition that is fighting the Islamic State."
Five people believed to be linked to Lahouaiej-Bouhlel, including his estranged wife, are in police custody, the Paris prosecutor's office said.