Arsenal defeated Chelsea for the first time since February 2004 to reclaim top spot in the Premier League.
Gallas (left) scored from a Fabregas corner late in the first-half
Manchester United had moved above them earlier in the day but William Gallas's header after Petr Cech failed to collect a corner ensured a Gunners win.
Shaun Wright-Phillips should have equalised but missed from six yards, while Manuel Almunia saved an Andriy Shevchenko free-kick in injury time.
Robin van Persie missed from 14 yards and was then denied by Cech late on.
Arsenal did not beat Chelsea during Jose Mourinho's entire tenure as Blues manager but have defeated Avram Grant's team at the first attempt.
Chelsea had gone 16 games without defeat in all competitions under Grant but now trail the Gunners by six points and United by five at the top of the league.
As with United's match at Liverpool earlier in the day, it was a tight game with little to choose between the teams.
But Arsenal showed a little more invention and ability on the break than Chelsea, who crucially lost John Terry to a suspected ligament injury after 38 minutes.
Even so, Chelsea should have equalised through Wright-Phillips, who scuffed his shot from six yards after Gallas had diverted a cross into his path midway through the second half.
Arsenal started brightly, their crisp passing and intelligent movement heavily in evidence as the home team looked for an early goal.
They had Cesc Fabregas, Mathieu Flamini and Alexander Hleb back in their line-up but Chelsea, with their five-man midfield, soon ensured that space and time on the ball was at a premium.
And it looked as though the first half would remain goalless before Gallas, playing against his old team, struck in time added on.
Chelsea had lost Terry after he failed to recover from a challenge by Emmanuel Eboue and, without the skipper's commanding presence in the box, Gallas was able to nudge Tal Ben Haim off the ball and convert Fabregas' corner.
But culpability for the goal lay with the normally reliable Cech, who came off his line to clear the corner but failed to make any meaningful contact with the ball.
There was plenty of niggle between the two teams
Brazilian defender Alex had almost gifted Arsenal the lead earlier in the half when he tried to head the ball to Cech but only succeeded in looping it over the keeper and, thankfully for him, narrowly wide of the Chelsea goal.
It was a fractious game with plenty of niggle, two melees and 10 yellow cards.
Possession was surrendered far too cheaply for either team to find any kind of rhythm and for long spells it seemed to suit Chelsea, who forced first-half saves from Almunia with decent long-range strikes from Wright-Phillips and Shevchenko.
Almunia dived sharply to his right to prevent Obi from scoring his first goal of the season in the second half, while Wright-Phillips's horrible miss was a real let-off for the home team.
The game became increasingly stretched as Chelsea searched for an equaliser and Arsenal cut them open with a precise break that substitute Van Persie was unable to finish from 14 yards.
Van Persie had the ball in the net after another break but the goal was ruled out for offside and later saw another effort saved by Cech, who then denied Fabregas with a follow-up.
Shevchenko had a late chance for Chelsea with a free-kick but Almunia tipped the effort over his bar.
The final action saw Arsenal waste another chance on the break and Fabregas booked for a tackle on Ashley Cole, who was booed throughout the afternoon on his return to his former club.
Arsenal: Almunia, Sagna, Toure, Gallas, Clichy, Eboue (Van Persie 69), Flamini, Fabregas, Rosicky, Hleb (Silva 77), Adebayor (Bendtner 90).
Subs Not Used: Lehmann, Senderos.
Booked: Adebayor, Eboue, Flamini, Toure, Fabregas.
Goals: Gallas 45.
Chelsea: Cech, Ferreira, Alex, Terry (Ben-Haim 38), Ashley Cole, Wright-Phillips (Kalou 75), Obi, Makelele (Pizarro 65), Lampard, Joe Cole, Shevchenko.
Subs Not Used: Cudicini, Belletti.
Booked: Terry, Lampard, Joe Cole, Ben-Haim, Obi.
Ref: Alan Wiley (Staffordshire).