The man believed to have attacked and wounded two people with a meat cleaver on Friday is cooperating with the police and said he had targeted weekly satirical magazine Charlie Hebdo, a police source told Reuters.
The attack took place in front of a building where Islamist militants gunned down employees of Charlie Hebdo in 2015 because of the republication of cartoons depicting Prophet Mohammad.
It coincided with the start this month of the trial of 14 alleged accomplices in the Charlie Hebdo attack. The gunmen behind that attack killed 12 people.
A judicial source said the suspect's custody had been extended on Saturday morning. According to French law, he faces a formal investigation at the end of the process.
A suspected accomplice of the attacker was released in the early hours of Saturday while another person close to the suspected attacker and who could had been his former roommate in a hotel north of Paris has been arrested.
By midday Saturday, seven people remained in custody including the suspected attacker.
Police quickly detained the man suspected of carrying out the attack next to the steps of an opera house about 500 metres away.
The suspected attacker was from Pakistan and arrived in France three years ago as an unaccompanied minor, Interior Minister Gerald Darmanin said.
A second suspect was detained moments after the attack and prosecutors were trying to establish his relation to the attacker. He was released free of charge, the source said.
Charlie Hebdo vacated its offices after the 2015 attack and is now in a secret location. The building is now used by a television production company.
Two of the production company's staff, a man and a woman, were in the street having a cigarette break when they were attacked, according to prosecutors and a colleague of the victims.
After the 2015 attack on Charlie Hebdo, investigators said the militants had wanted to avenge the publication of cartoons depicting the Prophet Mohammad in the magazine. Charlie Hebdo republished the cartoons on the eve of the trial.
Al-Qaeda, the militant Islamist group that claimed responsibility for the 2015 attack, threatened to attack Charlie Hebdo again after it republished the cartoons.
France has experienced a wave of attacks by Islamist militants in the past few years. Bombings and shootings in November 2015 at the Bataclan theatre and sites around Paris killed 130 people, and in July 2016 an Islamist militant drove a truck through a crowd celebrating Bastille Day in Nice, killing 86.