Arsene Wenger's landmark day ended in embarrassment for the Arsenal manager and elation for Chelsea counterpart Jose Mourinho.
Referee Andre Marriner was left equally red-faced after dismissing the wrong Arsenal player in Chelsea's 6-0 win on Saturday — giving Kieran Gibbs a red card after Alex Oxlade-Chamberlain made a diving save with his hand on the goal-line.
The 15th-minute sending off was hardly a turning point in the derby, however, as Arsenal had already conceded twice inside seven minutes of Wenger's 1,000th match in charge. And Arsenal crumbled further against the leaders after Eden Hazard converted the penalty kick, dropping to fourth in the standings as their title rivals all won.
"It's one of the worst days (in my career)," Wenger, who uncharacteristically did not attend the post-match media conference, told BT Sport television. "It's over after 20 minutes and it's a long game after that."
Manchester City climbed above Arsenal into third place with a 5-0 victory over Fulham, while second-place Liverpool won 6-3 at Cardiff. It leaves Chelsea four points ahead having played one more than Liverpool and three more than City, which is six adrift.
The bottom three remains unchanged with Fulham in last place behind Cardiff and Sunderland, which lost 2-0 at Norwich.
Hull surged eight points clear of danger by beating struggler West Bromwich Albion 2-0, Everton moved ahead of Tottenham into fifth by beating Swansea 3-2, and Newcastle climbed to eighth by defeating Crystal Palace 1-0.
Chelsea raced in front at Stamford Bridge, with Andre Schuerrle setting up Samuel Eto'o to curl in the opener in the fifth minute and then scoring himself two minutes later to complete another rapid break.
After Hazard's penalty following the red-card confusion, Fernando Torres squared for Oscar to scoop in Chelsea's fourth just before half time. Oscar netted again in the 66th after a curling shot slipped through Wojciech Szczesny's hands, and the goalkeeper allowed Salah's shot to roll under him four minutes later to complete Mourinho's biggest win as Chelsea manager.
The margin of this defeat matched the previous low-point of Wenger's 18-year reign — an 8-2 loss at Manchester United. It was also Arsenal's third big away loss against its direct title rivals this season, following a 6-3 defeat at Manchester City and 5-1 drubbing at Liverpool.
City also hit five on Saturday, with Yaya Toure contributing a hat trick.
The first two came from penalties — in the 26th and 54th, both times after fouls by Fernando Amorebieta who was sent off after the second — and curled in the third in the 66th. Fernandinho and Martin Demichelis added late goals.
The most goals of the day came in south Wales as Liverpool twice came from behind.
Luis Suarez cancelled out Jordon Mutch's opener for Cardiff, the first of three goals for the Uruguay striker. Although Frazier Campbell restored Cardiff's lead, Martin Skrtel levelled and then put the visitors in front for the first time early in the second half.
Suarez added two more, and Daniel Sturridge also found the net, while Mutch headed in his second for Cardiff.