Defending champions PSG were surprisingly undone at Guingamp, losing 1-0 following Jeremy Pied's close-range header in the 11th minute at the Stade du Roudourou.
Guingamp - fresh from progressing to the round of 32 in the UEFA Europa League midweek - pounced on some slack PSG defending to take the lead.
Thibault Giresse's free-kick was knocked down in the area by Younousse Sankhare and Pied made a late run into the box to nod past goalkeeper Salvatore Sirigu.
That proved to be the difference as PSG crashed to their first Ligue 1 defeat since a 2-1 loss at home to Rennes in May.
PSG's loss meant Laurent Blanc's men could not rise to the summit following Marseille's 1-0 defeat to Monaco.
Monaco were missing Dimitar Berbatov and Ricardo Carvalho but Leonardo Jardim's side still got the job done thanks to Bernardo Silva, who struck with 23 minutes remaining at the Stade Louis II.
Marseille (38 points, plus 20) - who suffered their first league loss in four matches - remain a point clear of PSG (37 points, plus 19).
Lille extended their unbeaten streak to three games after seeing off 10-man Toulouse 3-0 at the Stade Pierre-Mauroy.
Toulouse were reduced to 10 men in the opening minute after Maxime Spano was sent off for fouling Divock Origi inside the penalty area, though Marko Basa's spot-kick was saved by Ali Ahamada.
Lille eventually made the breakthrough in the seventh minute, Djibril Sidibe poking the ball home.
The home side put the result beyond doubt in the second half, with Ryan Mendes doubling the lead five minutes after the break.
Nolan Roux completed the scoring 13 minutes from time after collecting a pass from Origi.
Meanwhile, fourth-placed Saint-Etienne were held to a goalless draw by Nice.
