Veteran midfielder Paul Scholes, 37 - who had come out of retirement in last weekend's FA Cup tie at rivals City - was on hand to turn home Wayne Rooney's cross just before the break to net his first United goal since August 2010.
Earlier, Rooney had seen his penalty saved by Adam Bogdan, but the Trotters were undone again in the second half.
Danny Welbeck converted a centre from Rooney and midfielder Michael Carrick (83) lashed in a third goal as the Red Devils recorded a comfortable win to turn up the heat on their neighbours, who play at Wigan on Monday night.
Third-placed Tottenham to also move level on points with City when they were held to a 1-1 draw by a resilient Wolves at White Hart Lane.
Steven Fletcher (23) had given the visitors a shock lead when he bundled the ball in after a header from Roger Johnson had been cleared off the line.
Spurs thought they were level in first-half stoppage time, but Emmanuel Adebayor was offside when he followed up Gareth Bale's shot.
The equaliser eventually came on 51 minutes, when Luka Modric smashed in a 20-yard effort, but despite plenty of endeavour, the home side had to be content with just a point.
Chelsea beat Sunderland 1-0 at Stamford Bridge through a first-half goal from Frank Lampard.
The veteran England midfielder - whom manager Andre Villas-Boas claims is not for sale "at any price" - diverted the ball into the net after Fernando Torres' shot had come back off the crossbar on 13 minutes.
Sunderland have improved under new boss Martin O'Neill and caused the Blues some problems. James McClean went close, but the best chances came in the dying moments when substitute Craig Gardner and Nicklas Bendtner both stabbed ball wide in front of goal.
Under-fire Blackburn manager Steve Kean guided his 10 men to a battling 3-1 win over Fulham at Ewood Park.
It looked to be heading for another testing afternoon when Rovers striker Ayegbeni Yakubu was sent off after 26 minutes for high tackle on Danny Murphy.
Fulham striker Bobby Zamora hit the post with a header, and Paul Robinson kept out Philippe Senderos' effort before Rovers snatched the lead in first-half stoppage time through a free-kick from Morten Gamst Pedersen.
It was 2-0 at the start of the second half when David Dunn got on the end of a sweeping counter attack.
Although Damien Duff (56) reduced the deficit with a goal against his former side, Rovers restored their two-goal cushion will 11 minutes left when Mauro Formica converted Steven Nzonzi's cross.
Norwich beat West Brom 2-1 at The Hawthorns.
The Canaries were on the back foot for much of the first half, with keeper John Ruddy making some good saves before James Morrison's goalbound effort was cleared off the line by Andrew Surman.
The Norwich midfielder then swept the visitors in front just before the break, converting Wes Hoolahan's pass.
The Baggies were level from the penalty spot through substitute Shane Long after a terrible over-the-top tackle from Daniel Ayala on Jerome Thomas, which resulted in only a yellow card.
However, the visitors then produced another breakaway goal when Wales striker Steve Morison headed home a floating cross from Grant Holt.
Aston Villa and Everton shared a 1-1 draw.
Darren Bent put the home side in front on 56 minutes, but Victor Anichebe (69) earned the Toffees a share of the spoils when he beat the offside trap to level.
Stoke frustrated Liverpool at Anfield, where the Reds again failed to make the most of their chances in a goalless encounter - which was a costly seventh home draw.