Probably the moment that summed it up was Samir Nasri's celebration after he had scored the fourth goal and threatened to make it the biggest walloping Manchester United have taken at the home of Manchester City since the famous 5-1 in September 1989.

Nasri was the player in the corresponding fixture last season who had faced accusations of cowardice after ducking out of the way of Robin van Persie's late, decisive free-kick. Now he could be seen shadow-boxing by the corner flag. City had outfought their opponents in every department, from A to Z, and the bruises will take a long time to heal for the team beaten to the point of humiliation.

Manuel Pellegrini's side won with such remarkable, carefree splendour it felt as though the team in United's colours had no actual plan or idea about what this fixture is supposed to signify.

David Moyes now knows how the pre-knighted Alex Ferguson felt on that day at Maine Road, almost a quarter of a century ago, when United's manager of the time talked about going home and lying on his bed with a pillow over his head. At the height of this ordeal, Moyes's name reverberated around the stadium. Unfortunately for him, it was the home crowd serenading his appointment as Ferguson's successor.

Moyes should brace himself for a lot of scrutiny over the coming days because this was a performance that exposed serious shortcomings. Yet nothing should detract from what an exhilarating display of attacking, incisive football it was from Pellegrini's team. They were faster to the ball almost every time, playing with great fluency and instinct.

Van Persie was missing for United with a groin injury, but that still goes only a small part of the way towards explaining the difference between the sides. At 4-0, with 40 minutes still to play, it was almost a surprise City stopped there. At that stage, United looked vulnerable to an even more chastening experience than the 6-1 at Old Trafford two years ago.

As it was, Wayne Rooney rounded off a late, overdue flurry from the away side by scoring with a free-kick that was totally out of keeping with the rest of United's performance. At least the champions did not totally crumple but ultimately it was a futile exercise, delaying their first spell of prolonged pressure until Sergio Agüero had scored twice, Yaya Touré had pinched another and Nasri had rounded off a ghastly five-minute spell for United at the start of the second half when David de Gea was beaten twice in quick succession.

That would usually be the point of the match, having gone in 2-0 down at the interval, when United would be expected to set about trying to add another story to their portfolio of exceptional comebacks. What actually happened strayed dangerously close to capitulation and the defending was extraordinary in its shoddiness. Agüero was completely unmarked for 3-0 and it was the same again for Nasri when he hooked in Jesús Navas's cross four minutes later.

They were goals that fitted the narrative of the first half. Every one will cause Moyes serious alarm when he examines the mistakes that preceded them.

City, in stark contrast, were buoyant, playing with great energy and all the qualities of movement and penetration their opponents were missing. They played like a team affronted to have passed the title of champions United's way last season. They harried and chased and, when they had the ball, they made great use of it. The rout began in the 16th minute with a moment of improvisational brilliance from Agüero. Antonio Valencia did not track Aleksandar Kolarov's overlapping run from left-back. Nasri's clever little flick left Chris Smalling exposed at right-back and Agüero, twisting his body and jutting out his left foot, managed to apply just the right measure of control to volley in Kolarov's cross.

Valencia had committed one of football's oldest sins, as Nemanja Vidic forcibly let him know, but it was that kind of basic carelessness that continued to undermine United and their captain was just as culpable as anyone.

The tone was set early on when Vidic broke the first rule of defending, over-playing inside his own penalty area and losing the ball with a risky pass. Marouane Fellaini and Michael Carrick, the two players who were supposed to be shielding United's defence, were overwhelmed by Fernandinho and Touré.

Fellaini looked anything but a £27m player and, in the wide positions, there was a marked difference between the way Navas and Nasri pinned their markers back and the ineffectual offerings of Valencia and Ashley Young. At one stage, Young tried to clip a pass into the penalty area. It went straight out for a goalkick and, soon afterwards, City had their second goal.

This time it came from a corner that Nasri aimed for Alvaro Negredo, close to the penalty spot. Smalling tried to intercept but managed only to impede Vidic's jump. Fellaini was the third United player trying, and failing, to get to the ball and in the process he completely let Touré go. Charging in at the far post, Touré was a yard out as he applied the final touch from Negredo's knockdown.

Young, still managing to stand out for his wretchedness, was guilty late on of losing the ball in the build-up to City's fourth goal and Moyes, finally, had seen enough, substituting him straight away. Navas ran half the length of the pitch, unchallenged, before crossing for Nasri and the Frenchman's finish was wonderfully controlled on the volley.

A few minutes earlier, Nasri had played in Negredo, Vidic was spun far too easily and a simple cross left Agüero with a straightforward finish for the softest goal of the lot. United had been dismal to the core and Rooney's lack of celebration, after a marvellous free-kick, summed it up.

Man of the match Yaya Touré (Manchester City)