Three French policemen are being questioned for allegedly leaking security footage of an incident in which a presidential aide beat up a demonstrator in May, French media say.
This week Alexandre Benalla was exposed as the attacker shown on a video of the assault taken by a protester.
The officers are suspected of releasing security images to Mr Benalla to try to prove his innocence, the reports say.
The French presidency has been accused of failing to act swiftly against him.
Interior Minister Gérard Collomb has been summoned to answer questions in the National Assembly on Monday.
The three policemen taken into custody on Saturday are senior members of the Paris public order directorate, AFP news agency reports.
They were suspended on Friday.
What about Mr Benalla?
He has been sacked by the presidency. On Friday, President Emmanuel Macron’s office said it had made the decision after “new facts” emerged in the case.
Mr Benalla is being questioned over the footage of a woman and a man being beaten during May Day protests.
The video was posted on social media at the time. But the case became a political scandal after Le Monde newspaper revealed that the attacker, who is shown wearing a police helmet, was Mr Benalla.
A former bodyguard of Mr Macron, he was hired as an aide to the president’s chief of staff after last year’s election.
Mr Benalla faces accusations of assault and impersonating a police officer.
In May, a few days after the incident, he was given a two-week suspension, but nothing was reported to prosecutors.
The BBC’s Hugh Schofield in Paris says this suggests that Mr Macron’s office may have already been aware of his actions.
What happened on May Day?
The incident took place in a popular tourist spot in Paris’s Latin Quarter where about 100 people had gathered.
The original video shows a man wearing a police helmet, but not in uniform, joining CRS riot police after clashes erupted.
He grabs a woman by the neck, charging her down the street, before both disappear off camera.
Shortly afterwards he returns to the scene, attacking another protester who had been carried a short distance by police before being left alone on the ground.
The man in the helmet can be seen grabbing the young protester around the neck, hitting him on the head and apparently stamping on his stomach when he falls to the ground.