French police have detained a total of seven people following an attack with a meat cleaver that left two wounded outside the former offices of satirical weekly Charlie Hebdo in Paris, according to judicial sources.

The assault came three weeks into the trial of suspected accomplices in the 2015 massacre of the newspaper's staff.

Last night French police released another man who was close to the scene of the attack but who was confirmed to have been a witness who had "chased the assailant", the judicial source said.

Detainees include a former flatmate of a man arrested over the incident.

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

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

French Interior Minister Gerald Darmanin said 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.

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

Paris police said two people were "critically wounded" in yesterday'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 that a large meat cleaver found near the scene was used by the attacker.

The judicial source said today 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 last night.