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 after Mohamed Lahouaiej-Bouhlel ploughed a truck into crowds celebrating Bastille Day in the southern French city.
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.
Mr Sarkozy, a former president, called for any foreign nationals with links to radical Islam to be expelled from France.
Authorities say Lahouaiej-Bouhlel, a Tunisian living in Nice who carried out the mass killing last week, became radicalised only recently.
Investigators say they are yet to find evidence that he pledged allegiance to radical groups or had contact with known extremists, although Islamic State have claimed responsibility for the attack.
However, Paris prosecutor Francois Molins said images found on his computer as well as recent internet searches showed he had a "clear and recent interest" in radical Islamist movements.
Lahouaiej-Bouhlel rented the 19-tonne truck used in the attack on 4 July and drove to the Promenade des Anglais twice in the days before the attack, Mr Molins said.
Examinations of his computer showed he had searched for details of the Bastille Day celebrations in Nice as well as videos showing "terrible" traffic accidents.
This showed that his act was "premeditated and deliberate", Mr Molins said.
Many of the dead and injured were children watching a Bastille Day fireworks display with their families.
There are still 74 people in hospital, 28 of whom are in intensive care, Mr Molins said.
Thirteen of the victims have not yet been identified, he said.
Neighbours have described Lahouaiej-Bouhlel as a violent loner who liked to drink, lift weights and go salsa dancing.
France's Prime Minister Manuel 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.
Tunisian security sources have told the BBC he visited Tunisia frequently, most recently eight months ago.
Islamic State 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.
They are people who may have been in recent contact with Lahouaiej-Bouhlel or who may have helped him get the gun he used in the attack, Mr Molins said.
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.