Five people believed to be linked to the man who killed 84 people in Nice are in police custody.
The Paris prosecutor’s office says three arrests were made on Saturday and two on Friday, including the man’s estranged wife.
Mohamed Lahouaiej-Bouhlel drove a lorry into crowds marking Bastille Day on the Promenade des Anglais on Thursday before he was shot dead by police.
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.”
French President Francois Hollande met with his defence and security chiefs and cabinet ministers on Saturday.
He called for national unity in France and said: “We are in a time when, and we have seen it, there is a temptation to divide the country.
“Faced with these temptations, faced with this risk, we must recall the unity and cohesion of this country.”
Mr Hollande, who says the attack was a terrorist act, has already moved to extend a state of emergency by three months.
Prosecutors said a 31-year-old Tunisian Lahouaiej-Bouhlel drove the lorry 2km (1.2 miles) along the promenade targeting people.
Of the 84 who died, 10 were children. Some 202 people were injured; 52 are critical, of whom 25 are on life support.
Stephanie Simpson, from the Lenval children’s hospital in Nice, said five children remained in critical condition, one was in a “very bad” condition, three were on artificial respiration, one had been stabilised and one eight-year-old child remained unidentified.
Mr Hollande said 50 people were “between life and death”, while several people are missing and a “small number” of Britons were injured.
Reversed his decision
A state of emergency has been in place across France since the Paris attacks carried out by militants from the so-called Islamic State group. The 13 November attacks left 130 people dead.
Mr Hollande had proposed lifting the state of emergency on 26 July, but reversed his decision after the Nice attack.
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.
There was a visible security presence in Nice on Saturday morning and soldiers were patrolling the front of the main train station Gare De Nice Ville.
Stallholder Romain Ribero said France was used to high security in the wake of last year’s Paris attacks.
“We feel safe. My children and my wife live here. We feel secure. I am already hurt after Paris, but we must go on,” said the 37-year-old, who lost two friends in the November shootings.
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,” prosecutor Francois Molins said.
However, Prime Minister Manuel Valls said he was “in way or another” linked to radical Islam and Paris prosecutor Francois Molins said the attack bore the hallmarks of jihadist terrorism.
French Interior Minister Bernard Cazeneuve said the man responsible for the attack seemed to have been “radicalised very quickly”.
He said the “new type of attack… showed the extreme difficulty of the fight against terrorism”.
A neighbour of Lahouaiej-Bouhlel, who used to live in a high-rise block of flats on Boulevard Henri Sappia with his family, said he did not believe the 31-year-old was involved with IS.
Samiq, who did not want to give his surname, told the Press Association news agency: “I never saw him going to the Mosque. He was not a Muslim. During Ramadan I saw him smoking.”
Speaking in Nice, the president of the Regional Council for the Muslim Faith, Boubakar Bekri, said mosques in the area had responded to the attack.
“Yesterday in all mosques in the region of Alpes-Maritimes, there has been a common prayer calling for vigilance and patience, because these very bad events affect us; and there has been a call for blood donation,” he said.