The 37-year-old policeman was returning to his car after spending the evening with friends in Kingersheim, north of Mulhouse, according to press reports.
The Dernières Nouvelles d’Alsace said on its website that the man was beaten up and placed in his car, which the group then attempted set on fire.
The officer received head injuries and burns to his hands, the newspaper said, adding that the attack had all the appearances of a planned ambush.
The police union Alliance issued a statement saying the victim was clearly singled out for his role as an officer, even though he was off duty.
“The circumstances of this aggression do not leave any doubt about the fact he was targeted in his capacity as a policeman,” the union said.
“Our colleague was adamant, they cried “dirty cop” while they were attacking him,” a spokesman for the union, Michel Corriaux, is quoted as saying.
The policeman suffered “multiple bloody wounds to the face as well as a facial fracture, Alliance said.
The culprits have yet to be tracked down.
The union noted that the incident marked the latest in a series of attacks on officers in the Upper Rhine region of France.
It called for the “setting in motion of all the means to identify, arrest and bring to justice the authors of this criminal act”.