Arsenal earned an important win in a pulsating London derby against Chelsea to keep themselves in the race for the Premier League top four.

The victory moves the Gunners to within three points of fourth-placed Chelsea and back above Manchester United, who had gone fifth by beating Brighton.

Alexandre Lacazette put Arsenal ahead in the 14th minute when he made the most of a poor Hector Bellerin cross with a fine first touch before turning and beating Chelsea goalkeeper Kepa Arrizabalaga at his near post with a fierce strike.

Arsenal's intense pressing hampered Chelsea's ability to play the ball from the back, particularly in the first half, and the Gunners deservedly added a second through captain Laurent Koscielny in the 39th minute.

Chelsea failed to mark well enough at another set-piece and Koscielny missed a free header but the ball fortuitously looped in off his shoulder.

Maurizio Sarri's Chelsea struggled to create in the first half but did hit the post on the stroke of half-time through Marcos Alonso's header at a corner.

They posed greater threat in the second half but failed to have a shot on target until the 82nd minute as Arsenal's defence, led by the excellent Koscielny, held firm.