Chelsea spoilt Arsene Wenger's 1,000th match as Arsenal manager by humiliating their London rivals 6-0 at Stamford Bridge on Saturday to pull seven points clear in the Premier League.
Wenger was hoping to celebrate his milestone by cutting Chelsea's lead to a point, but instead his side were blown away by a flurry of first-half goals from Samuel Eto'o, Andre Schurrle, Eden Hazard and Oscar.
Hazard's goal came from the penalty spot after Arsenal midfielder Alex Oxlade-Chamberlain had handled the ball on the line, but a bizarre case of mistaken identity meant that his team-mate Kieran Gibbs was sent off instead.
Oscar completed his brace in the 66th minute before substitute Mohamed Salah scored to seal the club's biggest-ever win over Arsenal and give manager Jose Mourinho his most emphatic Chelsea success.
"This defeat is my fault. I take full responsibility for it," said a crestfallen Wenger.
"I don't think there's too much need to talk about the mistakes we made. We got a good hiding today."
Mourinho, whose side had crashed to a 1-0 defeat at Aston Villa last weekend, said: "At 2-0 after seven minutes, the game is over. We were too strong.
"We had momentum, but it stopped last Saturday against Aston Villa for some reason. Now we are trying to build more momentum."
Arsenal retain a game in hand on Chelsea and have the consolation of an FA Cup semi-final to look forward to, but they are now at risk of being left behind in the title race.
Liverpool can move to within four points of Chelsea with victory at second-bottom Cardiff City later on Saturday, while fourth-place Manchester City will climb above Arsenal, with two games in hand, if they beat bottom club Fulham.
'Ref, it was me!'
Arsenal were the first team to threaten at a sun-soaked Stamford Bridge, with Olivier Giroud forcing Petr Cech to make a smart save in the fourth minute, but from then on it was one-way traffic.
Eto'o opened the scoring a minute later, cutting inside Oxlade-Chamberlain and curling home, before Schurrle made it 2-0 two minutes later with a drilled shot into the bottom-left corner.
Eto'o was then forced off due to an apparent hamstring injury, with Fernando Torres entering the fray, but Chelsea continued to attack.
The 15th minute witnessed the game's most peculiar incident, as a diving Oxlade-Chamberlain used his left hand to block a shot from Hazard that was drifting narrowly wide of the Arsenal goal.
Referee Andre Marriner pointed to the spot, but sent off Gibbs, rather than Oxlade-Chamberlain, who could be seen on television vainly telling the official: "Ref, it was me!"
Hazard coolly lifted his spot-kick down the centre of the goal and there was even more misery to come for Wenger before half-time as Oscar tapped in Torres's low cross to further swell Chelsea's lead.
It was the second time this season that Arsenal had gone 4-0 down in the first half, after their 5-1 loss at Liverpool last month, and the second period brought yet more misery for Wenger's beleaguered side.
Oscar beat goalkeeper Wojciech Szczesny with a shot from outside the box before Salah ran through and slotted home his first goal since his January move from Basel.
Elsewhere on Saturday, seventh-place Manchester United will hope to harness the momentum from their 3-0 victory over Olympiakos in the Champions League in mid-week when they visit West Ham United.
Third-from-bottom Sunderland visit Norwich City and fourth-bottom Crystal Palace travel to Newcastle United, while Hull City host West Bromwich Albion and Everton tackle Swansea City at Goodison Park.