Pyotr Pavlensky's girlfriend was arrested on Saturday evening on charges of invasion of privacy and publishing images of a sexual nature without consent, the Paris prosecutor's office said.
Her arrest stemmed from the release by Pavlensky, a protest artist best known before now for nailing his scrotum to Red Square in Moscow, of a video of a man masturbating accompanied by screen grabs of racy text messages sent to a woman.
She has been widely named in French media as Alexandra De Taddeo, and is reportedly the former girlfriend of Griveaux and the person to whom he sent the video.
The footage is believed to have been recorded two years ago during her relationship with married father-of-three Griveaux.
She later began a relationship with Pavlensky, who has taken public responsibility for distributing the footage.
Griveaux dropped out of the race on Friday in the wake of the video's release, saying he wanted to protect his family.
Pavlensky, who claimed political asylum in France in 2017, was detained on Saturday.
Investigators said his arrest was over a fight at a Paris apartment on December 31st in which two guests apparently suffered stab wounds and he is accused of wielding a knife.
However, they may also question him over the sex tape.
On Friday, Pavlensky told AFP on Friday that he had posted the footage online in order to expose the “hypocrisy” of 42-year-old Griveaux and planned to post more material on a newly created “political porn platform”.
Griveaux “is someone who constantly brings up family values, who says he wants to be the mayor of families and always cites his wife and children as an example.
“But he is doing the opposite,” he told the Libération daily newspaper.
Politicians from across the French spectrum took a different view of the matter, however, calling the video an intolerable invasion of Griveaux's privacy.
Macron's LREM, which is poised for a drubbing in the municipal elections according to opinion polls, was left scrambling to find another candidate to try topple Paris Mayor Anne Hidalgo, a Socialist. On Monday they announced that the new candidate would be former health minister Agnès Buzyn.
Pavlensky has a track record of causing outrage.
His Red Square protest made global headlines in 2013 and two years later doused the doors of the Russian FSB secret police headquarters with petrol and set them on fire.
In October 2017, he set fire to the offices of the Bank of France on Place Bastille, site of the attack on an infamous prison at the start of the French revolution in 1789 – later receiving a jail sentence for the stunt.