Manchester United staged a stunning comeback at the Etihad Stadium as Amad Diallo's last-minute goal clinched a 2-1 win over crisis-hit Manchester City.
Ruben Amorim's side were trailing to Josko Gvardiol's first-half header until the 88th minute, when a tepid derby exploded into life as Matheus Nunes fouled Amad, following a poor backpass, to concede a penalty which was converted by Bruno Fernandes.
Moments later, the excellent Amad was put through on goal from Lisandro Martinez's ball over the top, touching it around Ederson before steering a superb finish into the net from an acute angle to leave the home supporters in stunned silence.
The result deepens the crisis facing Manchester City boss Pep Guardiola, whose side have only won one of their last 11 games in all competitions, continuing the club's worst run of form since 2006.
For Manchester United, the victory, only their second in the Premier League under Amorim, came after two huge calls by the new boss, who left forwards Marcus Rashford and Alejandro Garnacho out of his squad, citing selection reasons.
Their absences saw Amad pushed into a more advanced role and he duly punished the out-of-sorts hosts for failing to add to Gvardiol's opener, which was headed home from Kevin De Bruyne's deflected cross from one of few chances in the first half.
It appeared Guardiola's side were going to stumble over the line to victory when Fernandes dinked a one-on-one chance wide after Amad had seen a flicked header pushed around the post by Ederson in the second period, but City's latest implosion allowed United to celebrate a remarkable victory.
Champions Manchester City are left in fifth place, behind Nottingham Forest in fourth, and nine points off leaders Liverpool, who have a game in hand.
Watch: The all-important goals
Fernandes lauds 'unstoppable' Amad
Bruno Fernandes hailed Manchester United match-winner Amad Diallo, telling Sky Sports: "He has been doing great things and there's a lot more to come.
"He's always active, always alive and this is why he gets the penalty and his goal. He's been brilliant but I shouldn't say much as we need to keep him going as we need him at this level.
"Like this, he is unstoppable.
"I'm pretty sure he can do this every week. We expect it. He knows the expectation when playing for Manchester United. He's getting his chance deservedly. When you do the right things at the right moments the rewards are here."
Silva: 'Stupid' Man City played like under-15s
Bernardo Silva did not hold back on Manchester City's collapse, telling Sky Sports: "We totally deserved what happened.
"At this level, if it's a game or two, you can say that they are being lucky. If it's 10 games, it's not about that.
"If in the 87th minute of a derby, you are winning 1-0 with a corner for your team and the ball ends up with your 'keeper and a penalty for them, my friend…
"If you make these kinds of stupid decisions with three or four minutes to go, you deserve to pay for that.
"If you look at the game, there was only one team that could win. But in the end, we lost, and it's not one game, it's a lot of games lately.
"We have to look at ourselves. It's not about luck. It's the decisions you make. Today, in the last minutes we played like under-15s and we paid the price."