Robin van Persie scored an injury-time free-kick as United beat City 3-2 in the Manchester derby at the Etihad Stadium to move six points clear at the top of the Premier League table.
A Wayne Rooney brace saw United go in 2-0 up at the break only for City to equalise with four minutes left as Pablo Zabaleta slammed home after Yaya Toure had earlier pulled one back.
But Van Persie's free-kick took a deflection off Samir Nasri and flew into the far corner to give United a dramatic win.
The game ended amid chaotic scenes as Rio Ferdinand seemed to be struck by a coin, with blood pouring from his head.
The late incident, which came after more than 90 minutes of heated exchanges and late challenges, is being investigated by the police and FA.
Roberto Mancini started Mario Balotelli alongside Sergio Aguero in attack, with Carlos Tevez left on the bench against his former side and Edin Dzeko again reduced to the role of impact sub. David Silva shrugged off a hamstring injury to start while Gael Clichy and Yaya Toure were back in action after injury and suspension respectively against Borussia Dortmund in midweek.
Tom Cleverley was passed fit to partner Michael Carrick in United's midfield, with Darren Fletcher missing altogether and veterans Paul Scholes and Ryan Giggs on the bench. Shinji Kagawa and Nemanja Vidic were not deemed fit.
City were the stronger of the two sides in a frantic opening featuring several reckless challenges.
Their first chance fell to Balotelli on 11 minutes, but the Italy star blazed well over after Silva's visionary ball found Clichy who centred from the left byline.
Rooney's first goal on 16 minutes turned the game on its head. Ashley Young, one of two wingers - with Antonio Valencia on the right - employed by Alex Ferguson, won a header on halfway and received the ball back from Van Persie before attacking the left flank and centring low. Rooney's side-footed strike rolled seemingly in slow-motion inside the post with Hart flat-footed.
United looked the more cohesive team unit but it was Aguero who found himself in front of goal on 23 minutes, battering his way through the challenges of several United defenders before curling a low shot that former Atletico Madrid team-mate David De Gea saved comfortably.
There was space galore for both sides. Valencia, hugely impressive for the Red Devils, gave Rafael room on the right to cross for Rooney to finish his second from just outside the six-yard box on 29 minutes.
There were no changes at half-time but when Balotelli tried a backheel four minutes into the second period Mancini had seen enough and barked at Tevez to get ready to come on.
On 58 minutes, with City's pressure beginning to tell, Nasri drilled a low cross to the far post but Ferdinand slid back to deny Aguero.
It could have been 3-0 on the hour mark as Van Persie, quiet for most of the match but involved in much understated link-play, rolled the ball inside on to his right foot and crashes a shot against the inside of the far post. Young finished the rebound, but was deemed marginally offside - it was close - when the initial shot was taken.
And within seconds City were back in it. Tevez's arrowing low drive was stopped by De Gea and the Spaniard again saved from Silva, from point-blank range. However Tevez laid back for Toure to sidefoot into the bottom corner.
Tevez, Aguero, Nasri and Silva all attacked inside an increasingly fraught United box, but Chris Smalling - on for Jonny Evans - Ferdinand and Patrice Evra were imperious. And when the Frenchman was put on his backside by Silva as the Spaniard cut inside, De Gea saved brilliantly by throwing up a shoulder with the ball cannoning off the bar.
'Super-sub' Edin Dzeko came on and within two minutes City were level, although he played no part in the goal. A corner was headed out by Van Persie at the near post and Zabaleta arrived to smash a half-volley into the corner from 18 yards.
Yet it was not over. Tevez was angered when penalised for fighting for the ball inside the United box and booked for dissent. Shortly afterwards he flew back to defend and clipped Rafael accidentally as he cut in from the right.
He escaped a second booking but it gave United a chance with the free-kick - and Van Persie bent it inside the far post via the toe of Nasri's boot two minutes into injury-time.
Nasri may be criticised for shirking his blocking duties as he turned away on the end of the wall.
As United's fans celebrated and City's were enraged, Ferdinand was hit by what seemed to be a coin and after a stoppage the game was finally allowed to end.
There was still time for Tevez to boot Carrick up the backside in front of Alex Ferguson - and somehow escape a second yellow card - before United sealed the win.
MAN OF THE MATCH
Wayne Rooney (Manchester United) - His clinical first-half brace set the tone for the win, although Van Persie stole the headlines.
Manchester City: Hart 6; Zabaleta 8, Nastasic 6, Kompany 6, Clichy 6; Nasri 6, Barry 5, Yaya Toure 7, Silva 7; Aguero 7, Balotelli 5. Subs: Kolo Toure 5, Tevez 7, Dzeko 6.
Manchester United: De Gea 8; Rafael 6, Ferdinand 7, Evans 7, Evra 7; Valencia 8, Cleverley 7, Carrick 6, Young 8; Rooney 9, Van Persie 8. Subs: Jones 6, Smalling 8, Welbeck 6.