The 28-year-old male officer suffered serious burns and is in an induced coma, prosecutor Eric Lallement said.
His female partner was also badly burned on her hands and face.
The attack in the town of Viry-Chatillon was a "murder attempt", a police union official told RTL radio.
The officer with the most serious injuries was "between life and death", said Claude Carillo from the Alliance 91 union.
The officers were on patrol near a housing estate. Robbers have been known to target motorists at a traffic junction in the area.
A group of about 15 people were involved in the attack and surrounded the patrol car, the AFP news agency reports.
Two other officers who arrived as back-up were also injured.
Prime Minister Manuel Valls said those responsible would be "relentlessly pursued and brought to justice".
A French police officer is in a life-threatening condition a day after a group of youths pelted petrol bombs at his patrol car near a housing estate outside Paris, prosecutors say.