The Gunners moved up to second in the table above Manchester City on goal difference. United, on 37 points, are two points ahead of both sides, with a game in hand over Arsenal, having played two games fewer than City.
Chelsea are in free-fall – without a win in six games, a run stretching back to 10 November – after a solid start to the campaign. They remain six points behind United, but Sir Alex's men could stretch that lead to nine points with victory over Birmingham on Tuesday before Chelsea entertain Bolton at Stamford Bridge on Wednesday.
In North London, it was a game distinctly lacking in excitement until Alex Song put Arsenal in front two minutes before the break. Arsene Wenger's side were on the crest of a wave and after half-time twice struck on the break; Theo Walcott teed up Cesc Fabregas for Arsenal’s second, then the pair exchanged roles for Walcott to make it 3-0.
Branislav Ivanovic pulled a goal back for the visitors before the hour as Chelsea fought to claw their way back into the game. But the damage was already done.
There is, of course, a long way to go in a campaign that has proved more than capable of providing dramatic twists and seemingly unfeasible plotlines. But the aim for title contenders is to be in form and in touch with the leaders come the New Year. With one game left in 2010, and as the Reds go marching on, Chelsea look like being neither.