Match ends, Leicester City 4, Manchester City 2.
Jamie Vardy scored a hat-trick as Leicester City pulled off a stunning victory over an out-of-sorts Manchester City.
The Foxes were 2-0 up in five minutes as Vardy ended his 10-game run without a Premier League goal by slotting home, and Andy King curled in moments later.
It was 3-0 within 20 minutes as Vardy added another after skipping past visiting keeper Claudio Bravo, and he completed his hat-trick by intercepting a misplaced John Stones pass and finishing from a narrow angle.
Aleksandar Kolarov, with a free-kick, and Nolito scored late consolation goals for the away side.
They have now lost back-to-back league games for the first time since Pep Guardiola took over in the summer, and are four points adrift of leaders Arsenal.
For defending champions Leicester, victory ended a five-game run without a league win and moved them up to 14th.
Man City pay for chaotic defending
Guardiola said this week he wanted a new rule to allow teams to use up to six substitutes - and after five minutes he may have hoped his wish was a reality.
Manchester City have kept just two Premier League clean sheets this season, and Guardiola reshuffled his formation to start with a back three of Pablo Zabaleta, John Stones and Bacary Sagna.
That strategy was swiftly scrapped, though, when his side fell 2-0 down after just 255 seconds - Kolarov was hauled out of midfield and into a hastily created back four.
The visitors, looking marginally better for the change, still managed to get caught on the break for Leicester's third and frequently put themselves under pressure by trying to play out from the back - Stones' misplaced backpass for Vardy's third the costliest example.
Their fragility could well be a result of their lack of a consistent starting XI, with Guardiola never having kept the same line-up for two consecutive games during his time at Etihad Stadium.
Suspensions for Sergio Aguero, Fernandinho and Nicolas Otamendi had forced the Spaniard to make changes here, and his side were devoid of cohesion.
For all their possession, they did not have their first shot on target until Kolarov scored a well-placed free-kick in the 82nd minute, and only salvaged further respectability when Nolito tapped in the Serb's low cross late on.
Foxes show championship charm again
Leicester, who were a point above the relegation zone when they kicked off, were superb and, for perhaps the first time this season, showed all the hallmarks of last term's incredible title-winning campaign.
That most unlikely of championships was built on resolute defending, lightning-quick counter-attacking, Vardy's goals and Riyad Mahrez's magic - all of which were on show.
Mahrez pulled off a superb first touch to direct a high ball to Islam Slimani, who in turn slid in Vardy for the opener, and the Algerian repeated the feat when he redirected a long ball into the path of Vardy for Leicester's third.
Vardy, who scored 24 Premier League goals last season, had endured a 741-minute goal drought in the league, but took his tally for this term to five when he capitalised on Stones' error and somehow threaded a finish from the tightest of angles.
Man of the match - Jamie Vardy
'Welcome back Jamie' - what they said
Leicester boss Claudio Ranieri told BBC Sport: "It was the true Leicester, maybe because we have played so badly in our last few matches, but today we were so strong.
"We played smart, slowed down the tempo.
"I am very pleased for Jamie. When he finished, I said 'welcome back'."
Former Leicester defender Matt Elliott on BBC Radio Leicester: "There was a spark and zip about Leicester's play from the off. You could sense it, a freshness in the air.
"That's their best performance by some distance this season and that might just reignite their season, certainly in domestic terms."
Manchester City boss Pep Guardiola told BBC Sport: "Leicester won the second balls and scored fantastic goals.
"Football is a game with mistakes, especially today. I would never complain to my players and I will look inside myself and analyse the reason why we have problems with the second balls when they arrive.
"Our game is not bad, but in the box we have a lot of problems."
Match of the Day pundit Danny Murphy: "That's as good as I've seen Leicester this season, it was like the Leicester of last season. But City didn't half help them.
"Kolarov was supposed to be playing as a third centre-half but he played so far forward, he left Stones isolated. Zabaleta was playing in a weird in-between position and it didn't look like he knew what he was doing.
"Kevin de Bruyne appeared to be at left-wing-back. The communication and lack of line was pretty bad. Kolarov was playing his own game going forward so often. They gave Leicester so many opportunities to attack them. It could have been more.
"Fifty changes, they've made this season. It's nice to build relationships and have some familiarity. You can't keep changing players."
Defensive numbers make bad reading for Guardiola
- Guardiola has seen his side concede three or more goals in back-to-back league games for the first time as a manager.
- A Guardiola side conceded four goals in a league for only the third time (Atletico Madrid 4-3 Barcelona in 2009, Wolfsburg 4-1 Bayern Munich in 2015).
- Only Crystal Palace, Hull and Sunderland (one) have kept fewer clean sheets than Manchester City in the Premier League this season.
- Claudio Ranieri has won eight of his nine Premier League matches as a manager against Manchester City, drawing the other.
- Manchester City conceded twice in the opening five minutes of a Premier League game for the first time since October 2006 against Wigan.
On Monday, both sides find out who they will face in the last 16 of the Champions League.
Manchester City will be drawn against one of Atletico Madrid, Borussia Dortmund, Juventus, Monaco or Napoli.
Leicester City will meet Bayer Leverkusen, Bayern Munich, Benfica, Paris St-Germain, Real Madrid or Sevilla.
On Tuesday, Leicester travel to Bournemouth (19:45 GMT). Manchester City host Watford the following day (20:00).
- 17SimpsonBooked at 19mins
- 26MahrezBooked at 57minsSubstituted forJamesat 90+2'minutes
- 9VardySubstituted forGrayat 88'minutes
- 19SlimaniSubstituted forOkazakiat 78'minutesBooked at 90mins
- 6RegesBooked at 67mins
- 8GündoganSubstituted forNolitoat 68'minutes
- 15NavasSubstituted forSterlingat 58'minutes
- 72IheanachoSubstituted forY Touréat 58'minutes
- 17De Bruyne
- 42Y Touré
- Michael Oliver
- Shots on Target
Second Half ends, Leicester City 4, Manchester City 2.
Offside, Manchester City. David Silva tries a through ball, but Pablo Zabaleta is caught offside.
Attempt saved. Yaya Touré (Manchester City) right footed shot from outside the box is saved in the centre of the goal. Assisted by Pablo Zabaleta.
Attempt blocked. Nolito (Manchester City) header from the centre of the box is blocked. Assisted by Kevin De Bruyne with a cross.
Shinji Okazaki (Leicester City) is shown the yellow card for a bad foul.
Aleksandar Kolarov (Manchester City) wins a free kick in the defensive half.
Foul by Shinji Okazaki (Leicester City).
Substitution, Leicester City. Matthew James replaces Riyad Mahrez.
Attempt missed. Nolito (Manchester City) left footed shot from the centre of the box misses to the right. Assisted by Raheem Sterling.
Goal! Leicester City 4, Manchester City 2. Nolito (Manchester City) left footed shot from the centre of the box to the bottom right corner. Assisted by Aleksandar Kolarov.
Substitution, Leicester City. Demarai Gray replaces Jamie Vardy.
Foul by Raheem Sterling (Manchester City).
Riyad Mahrez (Leicester City) wins a free kick in the defensive half.
Corner, Manchester City. Conceded by Christian Fuchs.
Attempt saved. Nolito (Manchester City) header from the centre of the box is saved in the centre of the goal. Assisted by Pablo Zabaleta with a cross.
Pablo Zabaleta (Manchester City) wins a free kick in the defensive half.
Foul by Shinji Okazaki (Leicester City).
Goal! Leicester City 4, Manchester City 1. Aleksandar Kolarov (Manchester City) from a free kick with a left footed shot to the bottom right corner.
David Silva (Manchester City) wins a free kick in the attacking half.