At least four police officers were killed by a knife-wielding man before he was shot dead by police at their headquarters in central Paris on Thursday, France 24 reported.

The attack occurred at the Paris police headquarters on Île de la Cité, which is located across the street from the Notre Dame Cathedral.

A police union official identified as Loic Travers told AP that one police officer suffered fatal wounds and another officer shot the attacker “in a situation that appears to be self-defense”.

Another police union official, Jean-Marc Bailleul, told local french media that the attacker was reportedly a police department employee.

The attacker’s motives are not known yet. French Prime Minister Edouard Philippe and Interior Minister Christophe Castaner were at the scene to be briefed about the incident, BBC reported.

Paris Mayor Anne Hidalgo confirmed that multiple people were fatally wounded. “Paris cries for its own this afternoon after this terrible attack at the @prefpolice,” she wrote on Twitter. “Heavy casualties, several police officers lost their lives. On my behalf and that of Parisians, my first thoughts go to the families of the victims and their loved ones.”

“During the Paris Council, we will pay tribute to the victims and will salute the unfailing commitment of police forces serving the security of Parisians,” she added. “We know what we owe them.”

The attack came a day after thousands of police officers marched in Paris to protest against low wages, long hours and increasing suicides within their ranks.

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