Bradley Barcola got one goal and set up another as PSG moved 16 points ahead of second-placed Marseille, who conceded an added-time goal in a 1-0 home defeat to Socceroos goalkeeper Mat Ryan's Lens in the late game.
Third-placed Nice can move level on points with Marseille if they beat Lyon at home on Sunday.
Ryan, who joined Lens from Roma in the January transfer window, stopped a close-range volley from Brazilian winger Luis Henrique in the 83rd minute and moments later the man-of-the match made a superb one-handed save low to his right to deny forward Neal Maupay.
Marseille paid the price when the visitors launched a counterattack in the 94th and Colombia defender Deiver Machado teed up Neil El Aynaoui for a fine finish as Lens moved up to eighth.
Earlier on Saturday, PSG coach Luis Enrique rested captain Marquinhos ahead of the return leg at Liverpool in the last 16 of the Champions League next Tuesday, having lost the first leg 1-0 despite dominating.
Marquinhos was joined on the bench by striker Dembele, right back Achraf Hakimi, winger Khvicha Kvaratskhelia and goalkeeper Gianluigi Donnarumma.
Barcola put PSG ahead in the 28th minute when he placed the ball through goalkeeper Brice Samba's legs.
Barcola broke down the left again and picked out striker Goncalo Ramos in the 50th for 2-0. Three minutes later, sloppy defending allowed defender Lilian Brassier to pull one back and it galvanised Rennes, who came close to equalising several times.
Dembele's added-time brace took him this season to 20 league goals for the first time.
In other matches, Canada striker Jonathan David moved onto 14 league goals after scoring in Lille's hard-fought 1-0 win against rock-bottom Montpellier.
The win moved fifth-placed Lille level on 44 points with Monaco, who are ahead on goal difference, in the race for one of the four Champions League spots.
All the players had "WO=MAN" written on the back of their jerseys instead of their names as part of the French league's initiative centred around International Women's Day.
With AAP.
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