French Prime Minister Manuel Valls has been booed as he attended a minute's silence in Nice, where an attacker in a lorry killed 84 people on Thursday.
Hecklers shouted out "murderer" and "resign" at him before the minute's silence, held across the nation.
Earlier, centre-right opposition leader Nicolas Sarkozy accused the government of failing to provide security.
Troops are to be redeployed to tourist spots as inquiries continue into the killer's possible links to jihadists.
Sarkozy, a former president, called for any foreign nationals with links to radical Islam to be expelled from France.
Interior Minister Bernard Cazeneuve said the investigation had not yet found evidence linking attacker Mohamed Lahouaiej Bouhlel to terrorist networks.
The authorities say Lahouaiej-Bouhlel, a Tunisian living in Nice, became radicalised only recently.
While the so-called Islamic State (IS) group claimed the attack, Lahouaiej-Bouhlel may not necessarily have coordinated it with a larger network, Cazeneuve said.
Many of the dead and injured were children watching a Bastille Day fireworks display with their families.
Dozens of people remain in hospital after the attack.
Neighbours have described Bouhlel as a violent loner who liked to drink, lift weights and go salsa dancing.
Valls has suggested the killer may have been radicalised too quickly to trigger the authorities' attention.
He was shot dead by police when his vehicle's path along the Promenade des Anglais was eventually halted.
French media say he researched the route in the days before the attack.
Lahouaiej-Bouhlel reportedly drove through the seafront promenade area of Nice on Tuesday and Wednesday in preparation.
Tunisian security sources have told the BBC he visited Tunisia frequently, most recently eight months ago.
IS said the attacker was acting in response to its calls to target civilians in countries that are part of the coalition ranged against it.
Six people are being held in connection with the killings.
The latest arrests, of an Albanian couple who have not been identified, were on Sunday morning, French judicial sources say.
Lahouaiej-Bouhlel's estranged wife, who was detained on Friday, was released on Sunday.