Two police officers suffered serious burn injuries after an attack with Molotov cocktails in the town of Viry-Chatillon south of Paris.
About 15 attackers threw incendiary devices into two police cars Saturday afternoon, a spokesman for the police union told broadcaster BFMTV.
He said the attack was carried out by a "well-organized group," with which police are in a "civil guerrilla war."
The perpetrators fled the scene, according to Interior Minister Bernard Cazeneuve.
Two further police officers, who had come as reinforcement, were also mildly injured.