Man City moved into second place with a convincing derby win over United, with Arsenal denied crucial points in an own-goal draw against Swansea.
A clinical double from Edin Dzeko helped Manchester City move up to second place in the Premier League with a 3-0 win at Manchester United on Tuesday, while Arsenal's title hopes all but evaporated in a 2-2 draw with Swansea City.
Manchester City took the lead after 43 seconds at Old Trafford and never looked back, exposing United's soft centre almost at will with Dzeko and Yaya Toure netting in the second half to inflict another painful defeat on home boss David Moyes.
City, in ominous form after a 5-0 win over Fulham on Saturday, have moved into the slipstream of Chelsea and are three points behind the league leaders with two games in hand.
"It was an amazing game from the first to the last minute," striker Dzeko said on Sky Sports.
"We wanted to start playing our game early, to press high and get an early goal, which we did.
"After that it was all about us. We could have had another goal [in the first 10 minutes] but we scored two more in the second half."
Arsenal manager Arsene Wenger, like United's Moyes, faced difficult questions after his side conceded a 90th-minute own goal from Mathieu Flamini to draw with struggling visitors Swansea after a 6-0 thrashing from London rivals Chelsea.
It looked as though Arsenal would claim all three points when second-half goals within a minute from Lukas Podolski and Olivier Giroud cancelled out Wilfried Bony's opener.
Everton made it four wins in a row to maintain their push for Europe as they beat Newcastle United 3-0 away with England hopeful Ross Barkley, Belgium striker Romelu Lukaku and Leon Osman all on target.
Arsenal stay fourth on 63 points, two behind third-placed Liverpool who host Sunderland on Wednesday. Everton are up a place to fifth, six points behind Arsenal with a game in hand in the race for a top-four finish and Champions League football.
Manchester United should have approached the night buoyed by back-to-back wins, but instead began like a rabbit caught in the headlights of City's breathtaking early onslaught.
The second hand was still on its first lap of the clock when City took the lead, with Dzeko finishing into an open goal after a Samir Nasri shot hit the post.
United were in disarray as City, who have now won five of the past six derbies, continued to pile forward, with David Silva finding pockets of space in between the hosts' midfield and back line to weave his magic and keep United chasing shadows.
United could not stem the flow of City's attacks and Dzeko's second goal 11 minutes after halftime put the match to bed, with the Bosnian easily shaking off his marker Rio Ferdinand to steer a corner from the right in at the near post.
Toure administered the coup de grace in stoppage time when he drilled a low shot into the bottom corner.
"I think we have played a very good side, playing at the sort of level we are aspiring to," Moyes said after his second straight 3-0 home league loss following a defeat by Liverpool.
"We need to come up a couple of levels ourselves because at the moment we are not there."
After the embarrassment of Saturday's thrashing by Chelsea, the onus was on Arsenal to show they were still in the title race and were not going to be dragged into a dogfight for a top-four finish.
While the north London side have struggled against their top-four rivals, they have proved more than capable of dispatching the league's lesser lights.
But Swansea were up for a scrap and scented blood, taking the lead after 11 minutes when Bony directed a precise header into the net for his 11th league goal of the season.
The Swansea threat seemed to have passed when Podolski levelled in the 73rd minute with a volley and Giroud put the hosts ahead soon after by poking home from close range.
But with the clock ticking down calamity struck and Flamini, the player Wenger was criticised for not starting against Chelsea, deflected the ball inadvertently into his own net.
"You wonder how we could draw a game like that, they had nothing and in the end it was 2-2," a bemused Wenger said.
"At 2-1 we became too conservative because of what happened on Saturday and instead of continuing to attack it worries us more to keep the score."
Arsenal's slip means Everton might fancy their chances of chasing them down and finishing in the Champions League places.
They opened the scoring at St James' Park when Barkley was allowed to run unchallenged from inside his own half before firing past Tim Krul into the top corner after 22 minutes.
They doubled the lead when Gerard Deulofeu picked out Lukaku to poke the ball home seven minutes after halftime, and Osman then beat Krul at his near post to make it 3-0 late on. – Reuters