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They sent Blackburn crashing to their biggest defeat of the season with second half goals from Adam Johnson, Mario Balotelli and substitutes Samir Nasri and Stefan Savic who both scored their first goals for City.
It was their first match since their Champions League defeat at Bayern Munich on Tuesday when Carlos Tevez allegedly refused manager Roberto Mancini's instructions to come on as a substitute and their delight at the end was in stark contrast to the gloom that has hovered around the club since then.
Champions United also maintained their unbeaten start with a 2-0 home win over Norwich City thanks to second half goals from Anderson, who scored with a header, and Danny Welbeck which gave them a club record 19th successive home league win.
United, who were far from convincing and could easily have fallen behind to any number of Norwich chances including one that hit a post, stay top on goal difference from City with both teams taking 19 points from their opening seven games.
United have a goal difference of +19 compared with City's +18.
United manager Alex Ferguson agreed Norwich made it difficult for his side, telling Sky Sports: "I thought we were far too slow in the build up in the first half but we changed things around in the second and we dominated, but the thing about that kind of game is when you dominate and don't take your chances you're liable on the counter-attack, but we deserved to win it in the end."
In the day's lunchtime kickoff, Liverpool won 2-0 at Everton in the 185th Merseyside league derby with late goals from Andy Carroll and Luis Suarez. Everton played for more than an hour with 10 men after Jack Rodwell was sent off in the 23rd minute.
The dismissal was a controversial one because Rodwell played the ball cleanly and made minimal contact with Suarez as the momentum of his body carried him forwards, but referee Martin Atkinson showed him an instant red card.
Everton manager David Moyes said aftewards he was disappointed that a foul was given, never mind a sending off.
"It's not for me to criticise the referee, but he probably ruined this game of football today," he told Sky Sports.
Newcastle United maintained their surprisingly good start to the season to remain unbeaten after seven matches following their 2-1 win at Molineux to inflict a fourth consecutive league defeat on Wolverhampton Wanderers.
Newcastle won with goals from Demba Ba, his fourth in two games, and Jonas Gutierrez to climb to third after their best start in 17 years and they will stay there at least until Chelsea play at bottom-placed Bolton Wanderers on Sunday.
Wolves had a late goal ruled out when a linesman, wearing a cap to shade his eyes from the sun, ruled that the ball had gone out of play in the build-up, although it appeared he might have got his call wrong.
Aston Villa, also unbeaten with two wins and five draws from their opening games, sent Wigan Athletic to a fourth successive league defeat and down to 17th with a 2-0 win at Villa Park.
Gabriel Agbonlahor and Darren Bent were on target for Villa who ended the day in sixth place.
Sunderland drew 2-2 with West Bromwich Albion after trailing 2-0 inside the opening five minutes at the Stadium of Light.
Steve Morison and Shane Long gave West Brom the early initiative but Sunderland were level with two quick goals of their own midway through the first half from Nicklas Bendtner and Ahmed Elmohamady.
Manchester City were the highest scorers of the day with their four at Ewood Park, but their afternoon was marred when Argentine striker Sergio Aguero limped off in the first half with a groin injury.
He will now join his compatriot Tevez on the sidelines, but for totally different reasons following Tevez's suspension from the club for Tuesday's incident.
While the game ended well for City it was another unhappy afternoon for Blackburn, who are one off the bottom, with fans staying behind after the final whistle with banners and posters demanding the sacking of manager Steve Kean.
It was the second time in two weeks fans protested against the manager, but he told Sky Sports afterwards: "I hope they change because we are working hard.
"I know they are frustrated, we, the staff and players are as well, but I say, stick with us we will turn this around. We've got a lot of bright young players, but we are in a difficult moment."
Four matches are scheduled for Sunday with Bolton hosting Chelsea, Fulham playing Queens Park Rangers, Swansea City hosting Stoke City and Tottenham Hotspur facing Arsenal at White Hart Lane in the always eagerly-awaited north London derby.