Police have told ESPN FC an investigation has begun into allegations that officers knocked a Nice supporter unconscious, leaving him with a broken nose, at a Ligue 1 game last weekend.
The fan, named by newspaper Nice-Matin as Christophe B, was allegedly assaulted by riot police after arriving at Montpellier's stadium.
As well as the broken nose, he suffered another facial fracture around his eye and medical examinations showed he had been concussed in an incident that is now being looked at by France's police investigation unit.
"The police met us at a motorway rest stop and took us to the Stade de la Mosson," Christophe B told Nice-Matin.
"They were very professional and there was no problem. I was searched and had my little bottle of alcohol. A steward said to me: 'You're not going in with that, so if you want to drink it, you finish it here.' And so I did.
"During that time, another police officer did not stop insulting me and promised me I wouldn't get into the stadium, all of it gratuitous. I didn't answer him. Verbal provocation isn't a problem for me.
"The steward told me to stay where I was and only talk to him. So I threw away my bottle as I went in and the police officer insulted me again: 'Get out of here! You're not coming in.'
"It was then that I answered back and asked him to talk better to me. The policeman grabbed me by the throat and pushed me back several metres before pinning me to the fence. I found myself surrounded by 10 riot police.
"At the start, I let it happen, then I reacted. I tried to take his hand away, which was strangling me, and it was then I started taking punches. I couldn't move. I tried to defend myself. It lasted 30, 40 seconds before I lost consciousness."
The fan said he still had marks from the officers' shoes on his face and that there had been blood on his arm and face, as well as a "big pool" of it in front of him, when he regained consciousness.
After spending more than 30 hours in custody, he appeared before a court on Monday accused of aggravated assault by the police.
However, after the judge was shown video footage, he was released without charge and filed a complaint against the police with the support of Nice.
"During the hearing, the presiding judge saw a video and it appears almost certainly that he received blows he shouldn't have received," Montpellier prosecutor Christophe Barret said.