Cristiano Ronaldo inexplicably handled the ball from a Wayne Rooney corner
Cristiano Ronaldo was sent off as Wayne Rooney's 100th club goal proved enough to give Manchester United a derby win.
United were dominant early on but Manchester City came closest to scoring when Stephen Ireland grazed a post.
Rooney followed up Michael Carrick's shot to break the deadlock before Ronaldo was sent off for a second booking after inexplicably handling.
Richard Dunne almost levelled but saw his shot cleared off the line before Joe Hart kept out a late Rooney lob.
Hart, who had joined City's attack for an injury-time corner, ran the length of the pitch as United broke and got back to his goal just in time to deny Rooney his second goal of the game.
The home side had come close to equalising seconds earlier when Patrice Evra did brilliantly to block Dunne's effort but United were still convincing winners.
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Although they were against 10 men for the last 20 minutes following Ronaldo's dismissal, Dunne's shot was the only time City came close to breaking down a well-organised United defence in that period.
And Sir Alex Ferguson's side had been just as impressive going forward for the majority of the match.
The only blemish on an otherwise perfect day for Ferguson was Ronaldo's red card, which came after an inexplicable handball by the Portuguese winger.
Ronaldo had already been booked for bringing down Shaun Wright-Phillips and, when he almost caught Rooney's corner at the near post less than 10 minutes later, referee Howard Webb had no choice but to dismiss him.
It was a bizarre incident but did not take the shine off an impressive display by the visitors.
While City began nervously, United were into their stride from the start and could have been out of sight by the end of a one-sided first half.
Dimitar Berbatov, operating behind the marauding Rooney, tormented the City defence in the early stages, while Evra and Rafael da Silva were constant threats on either flank.
But, for all their possession, a goal continued to elude United.
Ronaldo nodded over from a corner, Hart brilliantly pushed a Berbatov header round the post and Evra blazed over when his own cross was cleared back to him in the box.
Rooney celebrates after scoring the 100th goal of his club career
It took City over half an hour to create any sort of opening but, when it came, they should really have taken the lead.
A whipped Javier Garrido free-kick was only cleared as far as Ireland, who fired the ball goalwards with United keeper Edwin van der Sar out of position.
Ireland's shot bounced towards Micah Richards, who was stood in front of the net, but instead of turning it home the defender left the ball and it hit the outside of the post.
That sparked City's best spell of the first-half, with Robinho inches away from collecting Didi Hamann's slide-rule pass in front of goal, but United remained dangerous and it was no surprise when they scored after 42 minutes.
The home side had plenty of opportunities to clear the ball but instead it fell for Carrick to fire in a low shot that Hart could only parry into the path of Rooney, who converted with glee for his 100th club goal.
Understandably, City boss Mark Hughes made changes at the break and, after moving Vincent Kompany into midfield, his side were instantly more competitive.
Hughes plays down Ronaldo dismissal
City were seeing more of the ball too but they still had to be alert to United's threat on the counter-attack and Ji-Sung Park was twice denied by last-ditch tackles by Wright-Phillips and Dunne.
Ferguson's side continued to get plenty of men behind the ball and even Ronaldo's dismissal did not help the home side, who looked to have run out of ideas before Dunne's late chance came and went.
The scoreline would have reflected the game far better had Rooney scored with his audacious injury-time lob and, in truth, the match perfectly illustrated the gulf in class between the two sides that City's wealthy new owners Abu Dhabi United are hoping to narrow.
Manchester City boss Mark Hughes on Ronaldo's red card: "I don't think the referee had any choice.
"It is a soft sending-off but it was a second bookable offence and he had to go.
"I don't know what reason he will give for doing it but if he says the ball was going to hit him in the face, why didn't he head it?"
Man City: Hart, Richards (Sturridge 76), Kompany, Dunne, Garrido, Wright-Phillips, Ireland, Hamann (Elano 46), Vassell (Zabaleta 46), Robinho, Mwaruwari.
Subs Not Used: Schmeichel, Ball, Jo, Ben-Haim.
Booked: Ireland, Vassell.
Man Utd: Van der Sar, Rafael Da Silva, Ferdinand, Vidic, Evra, Ronaldo, Carrick, Fletcher, Park (O'Shea 90), Rooney, Berbatov (Giggs 83).
Subs Not Used: Foster, Anderson, Nani, Evans, Tevez.
Sent Off: Ronaldo (68).
Booked: Rafael Da Silva, Fletcher, Ronaldo, Evra, Carrick.
Goals: Rooney 42.
Ref: Howard Webb (S Yorkshire).
BBC Sport Player Rater man of the match: Manchester United's Wayne Rooney 7.88 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.
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