French police have detained a former flatmate of a man arrested over an attack with a meat cleaver that left two wounded outside the former offices of satirical weekly Charlie Hebdo, a judicial source said Saturday.

Police also late Friday released another man who was close to the scene of the attack in Paris but who was confirmed to have been a witness who had "chased the assailant", the source said.

A total of seven people are now being held in connection with the attack, which came three weeks into the trial of suspected accomplices in the 2015 massacre of the newspaper's staff.

They include the "main perpetrator", who was arrested not far from the scene, prosecutors said.

France's PNAT specialist anti-terror prosecution office said Friday it had opened a probe into charges of "attempted murder related to a terrorist enterprise" against him as well as "conspiracy with terrorists."

According to PNAT head Jean-Francois Ricard, the suspect was an 18-year-old man. Initial indications were that he was born in Pakistan.

French Interior Minister Gerald Darmanin said Friday the attack was "clearly an act of Islamist terrorism".

Charlie Hebdo has angered many Muslims around the world by publishing cartoons of the Prophet Mohammed over the years, and in a defiant gesture reprinted some of the caricatures ahead of the trial.

- Meat cleaver found -

Twelve people, including some of France's most celebrated cartoonists, were killed in the attack on Charlie Hebdo by Islamist gunmen on January 7, 2015.

Paris police said two people were "critically wounded" in Friday's attack near the paper's former offices in the French capital's 11th district. The magazine's new address is kept secret.

A source close to the investigation confirmed to AFP on Friday that a large meat cleaver found near the scene was used by the attacker.

The judicial source said Saturday that a man was taken in for questioning who had been a flatmate of the main suspect when he was living in social housing in the Val-d'Oise department north of Paris.

Five more people arrested in the Paris suburb of Pantin during a search of a property linked to the main suspect were also being held, a judicial source said late Friday.

The Premieres Lignes news production agency said the wounded were its employees -- a man and a woman taking a cigarette break outside.

The company specialises in investigative reports and produces the prize-winning Cash Investigation programme.

The stabbing came during the trial of 14 alleged accomplices of brothers Said and Cherif Kouachi, the perpetrators of the 2015 attack on Charlie Hebdo that was claimed by a branch of Al-Qaeda.

A female police officer was killed a day later, followed the next day by the killing of four men in a hostage-taking at a Jewish supermarket by gunman Amedy Coulibaly.