A policewoman was brutally killed in a knife attack at a police station outside Paris on Friday, in what investigators are treating as a terrorist attack.

The attacker was reportedly shot by police and died of his wounds after striking the officer’s throat with a knife during the incident in Rambouillet, about 60 kilometres from the French capital.

The police officer died at the scene.

“In the fight against Islamist terrorism, we will not back down,” President Emmanuel Macron said.

Investigators took three people into custody after the attack, apparently acquaintances of the suspect.

It is common in terrorism investigations in France for people close to suspects to be taken into police custody for questioning.

Investigators searched two flats, including the suspect’s home.

Prime Minister Jean Castex, who visited the scene of the crime, said that France had lost an “everyday hero in a barbaric and entirely cowardly act.”

The sequence of events and the perpetrator’s statements were reasons why anti-terrorism investigators had taken over, prosecutor Jean-Francois Ricard said at the scene.

The police officer had been murdered “in a cowardly fashion,” he said, without giving any details.

The perpetrator is said to be a man from Tunisia, who was not previously known to the authorities.

He is said to have shouted “Allahu akbar” – “God is greatest” in Arabic – and to have been about 36 years old. The prosecutor did not comment on this either.

Mr Macron wrote on Twitter that the police officer was called Stephanie. He said the country stood by her family, her colleagues, and the police.

The media reported that the woman was surprised by the attacker when she entered the police station.

The attack allegedly took place in the entrance area, where the mother of two children had just returned from her break.

According to the reports, she was about 49 years old and had worked in the police station for years.

Jerome Moisant of the police union Unite SGP Police told Franceinfo that the deceased colleague had worked in administration.

The attacker had earlier been conspicuous outside the police station.

BFM TV reported that the attacker had been watching a jihadist video on his phone before the attack.

Interior Minister Gerald Darmanin instructed prefects in the country to strengthen the security around police stations. They are to increase vigilance and security measures in particular at the entrance areas.

For years, France has been rocked by Islamist attacks, killing more than 250 people.

In October, the teacher Samuel Paty was beheaded not far from Friday’s killing by an Islamist.

A short time later, an attacker struck at a church in Nice, killing three people there with a knife.