Four police officers were killed and five others wounded in a knife attack at Paris police headquarters on Thursday afternoon, the police union said.

The assailant, identified as an administration worker in the intelligence unit, stabbed to death four of his colleagues inside the police prefecture after 12 noon local time, Loic Travers, local head of the Alliance police union, said on BFMTV news channel.

Travers described the perpetrator as a model employee who had worked in the headquarters for two decades and had not shown unstable behavior until the attack.

The assailant's motive is not identified yet. But according to reports it could be related to a workplace dispute.

Paris Mayor Anne Hidalgo denounced the "frightful attack," which had left "heavy casualties."

"Several policemen lost their lives ... My first thoughts go to the families of the victims and their relatives," she tweeted.

President Emmanuel Macron visited the police headquarters shortly after the attack "to express his support and solidarity to all staff of the prefecture," his office said.

Prime Minister Edouard Philippe and Interior Minister Christophe Castaner, who had canceled a visit to Turkey, were also at the site.

The unprecedented assault inside the police prefecture came a day after about 27,000 police officers took to the streets to protest against low wages, long hours and increasing suicides in their ranks.