Four people believed to be linked to the man who killed 84 people in Nice are in police custody, French media say.
One of the men was arrested Friday and the three others were held on Saturday morning, AFP news agency reported.
Mohamed Lahouaiej-Bouhlel drove a lorry through crowds marking Bastille Day on the Promenade des Anglais on Thursday before he was shot dead by police.
French President Francois Hollande will chair crisis talks later.
Mr Hollande, who says the attack was a terrorist act, has already extended a state of emergency by three months.
Prosecutors said Tunisian Lahouaiej-Bouhlel droven the lorry 2km (1.2 miles) along the promenade targeting people.
Ten of the dead were children. Some 202 people were injured; 52 are critical, of whom 25 are on life support.
At the meeting with the security chiefs, Mr Hollande is expected to review all options in response to the attack.
A state of emergency has been in place across France since November’s Paris attacks carried out by militants from the so-called Islamic State group, in which 130 people died. It had been due to end on 26 July.
Some 30,000 people were on the Promenade des Anglais at the time of the attack, officials said.
Residents of Nice and foreign tourists were killed, among them four French citizens, three Algerians, a teacher and two schoolchildren from Germany, three Tunisians, two Swiss, two Americans, a Ukrainian, an Armenian and a Russian.
Mr Hollande said the attack was of “an undeniable terrorist nature”.
He warned that the battle against terrorism would be long, as France faced an enemy “that will continue to attack those people and those countries that count liberty as an essential value”.
Paris prosecutor Francois Molins said the attack bore the hallmarks of jihadist terrorism.
Lahouaiej-Bouhlel was known to the police as a petty criminal, but was “totally unknown to intelligence services… and was never flagged for signs of radicalisation,” the prosecutor added.