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".

26 July 2016 Last updated at 11:23 BST
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CCTV images of Francesca Galelli and Molly Curtis, both 13, were circulated by Cambridgeshire Police.
But the force issued a correction after it emerged the girls were not responsible for the theft from a branch of Superdrug in Ely.
Sgt Phil Priestley said police had made an "innocent mistake".
He said: "We always want to be clear and transparent if we get things wrong, and we definitely want to ensure that people know that Francesca and Molly are good people from lovely families, and this wasn't their fault".
Cambridgeshire Police have revised district CCTV policies to prevent such mistakes from happening in future.

Stanley Rangers RLFC said Evan Hawksworth was injured during a game on Sunday against Batley Boys.
The club said: "It is with the deepest regret that Stanley Rangers announce the tragic passing of their under-14s player Evan Hawksworth.
Evan's father, Gary Hawksworth, said: "Evan passed away playing the game he loved".
Under-14s team coach, Steve Abrahams, informed Evan's team-mates and described him as "our great friend, team-mate and all-round good guy."
Mr Hawksworth said: "He was never without a rugby ball in his hands.
"This was a tragic freak accident and no-one is to blame."
The club, with the RFL Benevolent Fund, has set up a crowdfunding page to support the family.
It said it would continue fundraising whilst celebrating Evan's life.
Events will take place at Stanley Rangers' ground on Friday.
A message on Evan's Facebook page said: "It is with sincere regret that today Evan Hawksworth has had his life support machine turned off.
"Words simply cannot portray the injustice of the loss of a charismatic, heart of a lion - tackle anything, be involved with everything, award winning young man".
Castleford Tigers posted on Facebook: "All our thoughts tonight are with Stanley Rangers U14s. We are saddened to let you know that Evan Hawksworth passed away today after sustaining an injury on Sunday. It was a freak injury playing the game he loved."

The cards, for the Scottish parliamentary election on 5 May, spell Scotland as "Scotalnd".
The error was not spotted until the official notification started to be delivered to voters in the city.
The authority said it accepted the mistake was "disappointing and embarrassing".
A spokeswoman for the Returning officer said: "This is a very disappointing, and of course, a very embarrassing proofing error.
"The 175,000 poll cards distributed will not be replaced as this unfortunate spelling mistake in no way invalidates the poll cards."

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19 November 2013 Last updated at 14:24 GMT
Engineers have used a tunnelling machine called Phyllis to dig through 6.8km (4.2m) of earth from Royal Oak in west London to Farringdon on the edge of the City.
BBC London's Chris Rogers took a look around the tunnel and spoke to the project manager of Western Tunnels, Andrew Alder.

Jordan Dawson, 19, from Guisborough, North Yorkshire, was killed in the crash, in February 2014, on the A174 Loftus bypass.
Cleveland Police said a 20-year-old man, also from Guisborough, had been charged earlier on Monday.
The man is due to appear at Teesside Magistrates' Court on 27 February.
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.
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