A man employed at the police headquarters in central Paris stabbed four officers to death on Thursday before being shot dead by police, officials said.
The attacker worked at the police building in an administrative capacity.
Union official Loic Travers said the attack appears to have begun in an office.
It continued elsewhere inside the large compound across the street from Notre Dame Cathedral, he said.
Travers said the motive behind the knifing was unknown, but the employee allegedly responsible for the violence was not known for posing any problems before.
He said he can’t remember an attack on police officers of this magnitude.
“Did he snap, or was there some other reason? It’s still too early to say,” Travers told BFM television.
At least one metro station in the vicinity of the building, which is close to Notre-Dame cathedral and other major tourist attractions, was closed.
An emergency message was broadcast over loudspeakers at the courthouse next door, announcing “an attack” at the police headquarters and stating the area was “under surveillance”.
Interior Minister Christophe Castaner, who was due to visit Turkey later on Thursday, postponed his trip to visit the scene of the attack.
“People were running everywhere, there was crying everywhere,” said Emery Siamandi, and interpreter who was in the building when the attack happened.
“I heard a shot, I gathered it was inside,” he told AFP. “Moments later, I saw police officers crying. They were in a panic.”
The Paris prosecutor is at the scene, but anti-terror agencies have not been involved at this stage.
France has been rocked since 2015 by a succession of attacks blamed on Daesh militants, which have included both large synchronised assaults and isolated knife and gun attacks, killing more than 250 people.
The country remains on high alert after these attacks.