Alexandre Benalla, an assistant to President Emmanuel Macron’s chief of staff, was filmed targeting a woman and a man during May Day protests.
He was caught on video by a student activist and left the scene once challenged on camera.
He was identified from the video by French newspaper Le Monde.
The incident took place in a popular tourist spot at Place de la Contrescarpe in the fifth district of Paris, where about 100 people had gathered on 1 May.
The original video, posted on social media by 21-year-old Taha Bouhafs, shows a man in a police helmet who is not in uniform joining CRS riot police after clashes erupted.
Skip Twitter post by @T_Bouhafs
???? ALERTA VIOLENCES POLICIÈRES
DES POLICIERS TABASSENT ET GAZENT TOUT LE MONDE PLACE CONTREESCARPE !!
FAITES TOURNER IL FAUT QUE TOUT LE MONDE VOIT !!#ViolencesPolicieres #1erMai pic.twitter.com/Dabr6HHwyJ
— Taha Bouhafs? (@T_Bouhafs) May 1, 2018
End of Twitter post by @T_Bouhafs
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, since identified as Mr Benalla, can be seen grabbing the young protester around the neck, hitting him in the head and apparently stamping on his stomach when he falls to the ground.
Riot police do not appear to intervene.
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A presidential spokesman said Mr Benalla had been given permission to attend the disturbance as an observer on his day off. Other photographs show him wearing a police armband.
Who is Alexandre Benalla?
Mr Benalla’s main duty is to arrange security for the president’s engagements.
Before he joined the presidential staff he had the role of head of security during Mr Macron’s election campaign in 2017.
Mr Macron and Mr Benalla in May 2017, when Mr Benalla was head of the candidate’s security
In that role, he was a constant companion to the future president, and archive photographs show the two men together at many high-profile public events.
But Richard Ferrand, a senior member of Macron’s party, sought to downplay the importance of Mr Benalla’s role.
“This is not a close aide, this is someone who was in charge particularly during the presidential campaign and then he joined the Elysée staff,” he told French TV.
President Macron, asked if he had confidence in his bodyguard on Wednesday night, pointed to a member of his entourage. “My bodyguard’s over there,” he said.
Skip Twitter post by @arthurberdah
VIDÉO – Interrogé par les journalistes sur #AlexandreBenalla, Emmanuel Macron refuse de répondre aux questions. Mais quand c’est un enfant qui lui demande s’il a “confiance en son garde du corps”, il se tourne, pointe son entourage du doigt et dit: “Il est là mon garde du corps!” pic.twitter.com/jGVEaV8lrA
— Arthur Berdah ⭐⭐ (@arthurberdah) July 18, 2018
End of Twitter post by @arthurberdah
Formerly an employee of a private security firm, he had worked with other French politicians in the past – including leading Socialist Martine Aubry and Mr Macron’s predecessor in the Elysée, François Hollande.
In 2012, he was hired as a driver for industry minister Arnaud Montebourg.
Mr Montebourg told Le Monde that Mr Benalla was fired for misconduct after causing a car accident in the minister’s presence and wanting to flee the scene.
What has the reaction been?
Elysée palace spokesman Bruno Roger-Petit said Mr Benalla had been suspended for two weeks without pay from 4 to 19 May, a punishment described as the heaviest so far meted out to a head of mission working at the presidential office.
He had also been moved out of his role of organising security for the president’s visits.
However, French TV reported that Mr Benalla had handled security for two key events this month, including the parade of the World Cup-winning France football team along the Champs-Elysées.
Paris prosecutors announced on Thursday they were opening a preliminary investigation into the alleged assault.
Possible charges include violence by a public official, pretending to be a policeman and the illegal use of police insignia.
Speaking to French radio on Thursday, activist Taha Bouhafs said that protesters had been “quietly settled” on the square before he recorded his video.
“The man on the ground was harmless and begged Benalla to stop,” Mr Bouhafs said. “There is no explanation for this outburst of violence.”
On the day of the protests, Mr Macron took to Twitter, vowing that those responsible for violence would be “identified and held accountable for their actions”. The tweet has received renewed attention in light of the accusations against his employee.
Skip Twitter post by @EmmanuelMacron
Je condamne avec une absolue fermeté les violences qui ont eu lieu aujourd’hui et qui ont dévoyé les cortèges du 1er mai. Tout sera fait pour que leurs auteurs soient identifiés et tenus responsables de leurs actes.
— Emmanuel Macron (@EmmanuelMacron) May 1, 2018
End of Twitter post by @EmmanuelMacron
Opposition MPs have called for far stronger action to be taken against the presidential official.
Socialist Olivier Faure complained of double standards, arguing that there was one form of justice for regular people, and another for the presidency.
Alexis Corbière of the far-left France Unbowed party called for Mr Benalla’s sacking, and said criminal charges should follow if the allegations were confirmed.