The incident happened on a bus carrying a Paris firefighter sports team back from a gymnastics competition in the eastern city of Colmar, Radio France Internationale reported Saturday.
The victim, whose name was not reported, was taken onto the bus, tripped, slapped and bitten on his buttocks. When he protested, the ringleader of the attack pinned him to the ground and raped him, the Le Parisien newspaper reported.
The 12 firefighters -- of France's elite Brigade de sapeurs-pompiers de Paris -- have been charged for the alleged rape. The squad captain and a trainer have been charged with "non-assistance to a person in danger" for allegedly allowing the incident to take place.
So far, only the leader of the attack has been taken into custody.