Robin van Persie scored a sensational stoppage-time winner to give Manchester United a 3-2 win at Manchester City on Sunday that sent them six points clear in the Premier League.
Pablo Zabaleta looked to have rescued a point for City with an 86th-minute equaliser but in the second minute of injury time, van Persie's free-kick flicked off Samir Nasri and crept in to put United in control of the title race.
Wayne Rooney had put the visitors 2-0 up with a first-half brace, before Yaya Toure replied for City on the hour.
There were unruly scenes in the aftermath of van Persie's winner, with United centre-back Rio Ferdinand left bleeding after appearing to be struck in the face by an object thrown from the crowd.
City substitute Carlos Tevez was also fortunate to escape censure for a wild kick at Phil Jones in the dying stages, but the final whistle prompted full-bodied celebrations from Alex Ferguson's visiting players.
"City always come back, we knew that, and it is a really special way to end this game in a dramatic way with the three points," van Persie told Sky Sports.
"Wazza (Rooney) asked if I was going to shoot or cross from the free-kick, and I chose to shoot.
"It was a great atmosphere and everyone was supporting their team. But what you saw happen to Rio should not happen on the football pitch."
City had not been beaten at home in the league since December 2010 and were protecting the last unbeaten record in English league football, but they now find their hold on the title slipping after a week that also saw them eliminated from all European competition.
"We won the league with the last touch of the ball last year," said City coach Roberto Mancini.
"This can happen. But we didn't deserve to lose this game. We played very well."
Mancini handed a surprise start to Mario Balotelli and he had City's first two openings, forcing David de Gea into a scrambling save with a low free-kick and then ballooning Gael Clichy's cutback over the bar.
United had not been near the hosts' area, but in the 16th minute they drew first blood.
Van Persie's chested lay-off freed Ashley Young down the left and his pass found Rooney, who cut inside Clichy before hitting a low shot that wrong-footed England colleague Joe Hart.
There was more woe for the hosts four minutes later when captain Vincent Kompany, scorer of the only goal in last season's corresponding fixture, had to go off with an apparent groin injury.
Hart had to race from his line to thwart Young and although Sergio Aguero then worked De Gea after barrelling through three challenges, the game's complexion had been altered and in the 29th minute, United extended their lead.
Rafael crossed from the right and with City's centre-backs distracted by van Persie's dart into the six-yard box, Rooney had time to sweep a right-foot shot into the bottom-left corner from 12 yards.
Rooney's brace made him the youngest player to reach 150 goals in the Premier League era and took this season's tally to seven in the league -- and four in two games -- despite recent criticism that he was losing his scoring touch.
Both sides made changes early in the second half, with Chris Smalling replacing Jonny Evans at centre-back for United and Tevez coming on for Balotelli, who stalked straight down the tunnel in trademark fashion.
Van Persie came within inches of making it 3-0 in the 59th minute, his glorious, bending effort coming back off the post before Young's tap-in was ruled out for offside despite television replays suggesting otherwise.
Seconds later City were back in the game, Yaya Toure rolling home from Tevez's cutback after De Gea had thwarted the substitute and then Silva in quick succession.
As the noise inside the Etihad Stadium increased, the drama swayed from end to end.
Patrice Evra had a penalty appeal turned down, van Persie headed over the bar, and moments later, De Gea produced a miraculous reaction save to divert Silva's ferocious shot onto the bar from close-range.
Zabaleta drove home from Tevez's corner to send the home fans into delirium but van Persie's last-ditch intervention ensured there was one final twist.