The damage was all done in the opening 20 minutes with the hosts too hot as the shell-shocked Blues conceded three quick-fire goals.
Jamie Vardy completed his hat-trick 12 minutes from time and a couple of late consolation goals reduced the arrears for City, but it was too little, too late.
Things couldn’t have started much worse for City as a pumped-up Leicester raced out of the blocks and within five minutes were two goals to the good.
Rarely have the Blues started a Premier League worse than this and even with 85 minutes still play, it was hard to see City leaving the King Power Stadium with anything other than a heavy defeat.
The defending champions, it appeared, had finally woken from a stupor that had left them in danger of dropping into the bottom three.
The opening goal came when Riyad Mahrez found Slimani who was afforded too much space in front of the Blues’ defence and his simple pass through to Jamie Vardy left the England striker clear to fire a low drive past Claudio Bravo.
Two minutes later a cross into the box was cleared to the edge of the City box where Andy King was waiting to power a 20-yard shot high past the despairing Bravo to double the Foxes’ lead.
Defensively, Guardiola’s side were simply unable to cope with the hosts’ early intensity, pace and counter-attacks.
On 20 minutes, it got even worse as the Blues were again caught by a long punt forward and Mahrez played a sublime pass into Vardy’s path and again the Foxes striker made no mistake, rounding Bravo and slipping the ball into the empty net.
Where, the travelling City fans must have wondered, had the Foxes been all season?
Shell-shocked, City took several minutes to compose themselves, trying to find a foothold by keeping possession and pushing Leicester back, but it looked as though the damage had already been done.
There were a couple of strong shouts for a penalty for the Blues before the break with Aleksandar Kolarov pulled down off the ball as he went for a header and then the same player – and John Stones – were both clearly held from another corner by Robert Huth and Wes Morgan – but the referee awarded neither.
City could only get better after the break - and did. The opening moments of the second-half were all City with Kelechi Iheanacho and Pablo Zabaleta going close, but an early goal was crucial if Guardiola’s men were to salvage anything from this fixture.
The Blues continued to knock at the Foxes’ door, dominating possession and going close through and Zabaleta before the hour-mark, but the breakthrough just wouldn’t come.
For all City's probing and prodding, it was Vardy who scored the next goal, pouncing on a poor Stones back pass to round Bravo and slide home from the narrowest of angles via the post.
Kolarov's free-kick four minutes later reduced the arrears and a Nolito strike on 90 minutes reduced the gap further, but it was scant consolation for a City side who had a very bad day at the office and will look to bounce back quickly against Watford next Wednesday.
Moment of the match
From City’s point of view, there wasn’t one. If there was a moment that decided the destiny of the points it was when Vardy was afforded enough room to burst through and open the scoring,
Milestones and landmarks
Kolarov made his 150th Premier League game for City today and Kelechi Iheanacho his 50th match in all competitions.
There are a couple more on the horizon, too.
Yaya Toure also moved on to 199 league games for the Blues.
City must return to winning ways against Watford on Wednesday. The wounds from this defeat will still be raw and Guardiola will look to use that pain and punish the Hornets - especially as Arsenal are the next opponents four days later...