Two people were seriously injured after they were stabbed in Paris near the former offices of the satirical magazine Charlie Hebdo.
Two suspects have been arrested. One of them was seized in the nearby Bastille area with blood on his clothing, police said.
Anti-terrorism police have taken over the investigation.
A security cordon has been set up in the 11th arrondissement in eastern Paris.
Nearby metro stations were closed and five schools in the area immediately went into lockdown. The schools were allowed to reopen some hours later.
A blade – described as a machete or a meat cleaver – was recovered at the scene of the attack near the Boulevard Richard-Lenoir.
Police quoted by French media said the “main suspect” was from Pakistan and the second person arrested was from Algeria. They have not yet been named and any connection between them has not been disclosed.
The conditions of the victims have not been made public although French Prime Minister Jean Castex told reporters at the scene that their lives were not in danger.
The two people wounded were staff at a TV production company, one of their colleagues.
“Two colleagues were smoking a cigarette outside the building, in the street. I heard shouting. I went to the window and saw one of my colleagues, covered in blood, being chased by a man with a machete in the street,” another member of staff at the Premières Lignes production firm said.
The firm has offices in the Rue Nicolas Appert, a side street off Boulevard Richard Lenoir where the former Charlie Hebdo offices are located. A mural to the 12 people killed in the Charlie Hebdo attack is nearby.
The satirical magazine has since moved to a secret location.
Mr Castex visited the scene accompanied by Interior Minister Gérald Darmanin and Paris Mayor Anne Hidalgo.
The prime minister reiterated the government’s “firm commitment to combat terrorism by all possible means”.
The attack comes as a high-profile trial is under way in Paris of 14 people accused of helping two jihadists carry out the 2015 attack on Charlie Hebdo, in which 12 people were killed.
Charlie Hebdo has marked the start of the trial by reprinting controversial cartoons of the Prophet Muhammad that sparked protests in several Muslim countries.
In response, the militant group al-Qaeda renewed its threat to the magazine.