Robin van Persie put Arsenal back into the Barclays Premier League title race with a superb second-half double to beat Chelsea at Stamford Bridge.
Van Persie struck twice in three minutes after the Blues had taken a first-half lead through a fortuitous Johan Djourou own goal.
But while the Dutch striker's second goal was sheer class, his equaliser was clearly scored from an offside position.
Chelsea remain top on goal difference from Liverpool but the Merseysiders will replace them if they get at least a point from their home clash with West Ham on Monday night.
The victory helped Arsenal to move back into the top four and cut the gap between them and Chelsea to seven points.
There was no hint of an Arsenal revival after a first half which saw them largely outplayed by the league leaders.
Arsenal's season, dogged by dressing-room unrest and five league defeats, looked in danger of imploding completely.
The opening 45 minutes was typically frenetic for a London derby and although Chelsea started brightly, the visitors almost snatched the lead in the 13th minute.
A cross to the far post from Emmanuel Adebayor was expertly controlled by Samir Nasri at the far post.
His drilled shot was only kept out by the outstretched arm of the giant Petr Cech in the Chelsea goal.
But Chelsea weathered Arsenal's early fire and almost opened the scoring themselves in the 23rd minute when a cross from Jose Bosingwa was met by the unmarked Frank Lampard.
The England midfielder sent his header goalwards only for Manuel Almunia to grasp it with both hands.
But Chelsea went ahead in the 29th minute thanks to some comedy football from Arsenal.
Almunia threw the ball straight to Bosingwa and when the Portugal right-back had exchanged passes with Nicolas Anelka, he sent over a dangerous low cross that Djourou could only turn into his own net from six yards.
But the first portent of an Arsenal revival arrived in the 32nd minute when Van Persie forced Cech into another fine save after the Arsenal midfielder had got the better of Branislav Ivanovic and unleashed a low drive towards the bottom corner.
Arsenal could have gone further behind five minutes before the interval but Almunia denied Salomon Kalou after Lampard's audacious headed flick had put him clear.
Chelsea had not conceded a second-half goal all season and so any chance of an Arsenal fightback was deemed remote at best.
The home side should have gone two in front four minutes after the restart but Lampard saw his shot flash just wide of Almunia's left-hand upright.
It was the last Chelsea attempt before Arsenal, against all the odds, turned the game on its head with two goals in three minutes.
The first was a controversial one as Van Persie was clearly yards offside when Denilson found him on the edge of the penalty area.
But with no whistle from referee Mike Dean, the Arsenal midfielder smashed the ball into the net for a 59th-minute equaliser.
Three minutes later the Dutchman put Arsenal in front. There seemed little danger as Van Persie shielded the ball on the edge of the penalty area with his back to Cech's goal.
But a superb shot on the turn sent the ball through the legs of Lampard and beyond the surprised Cech and into the bottom corner to the delight of the Arsenal fans. It was his ninth of the season.
Chelsea were stunned but the introductions of both Florent Malouda and Miroslav Stoch in place of John Obi Mikel and Deco, proved fruitless for Luiz Felipe Scolari.
Chelsea have not won at home now for three games and the flowing football that allowed them to destroy Sunderland 5-0 at the start of the month has deserted them
Scolari's side ran out of ideas as the game wore on and even when they fashioned a rare chance to get back in the game, it was squandered by poor finishing.
Malouda sent a shot high into the crowd with four minutes remaining when the Frenchman should have at least hit the target.
Arsenal were clearly in no mood to allow Chelsea to get back in the game and despite a spell of injury-time pressure, Wenger's side clung on to defy the critics and keep themselves in the hunt for the title.