Erling Haaland played a pivotal role in a Manchester Derby win once again
Erling Haaland was the main tormentor with goals in each half before setting up a third for Phil Foden as Pep Guardiola's side established superiority from the kick-off and rarely looked like relinquishing control.
There were poignant scenes before the start as supporters from both sides of Manchester's footballing divide joined to pay tribute to the late Sir Bobby Charlton.
For manager Erik ten Hag and his players, the match that followed was one to forget.
Haaland put City ahead from the penalty spot after 26 minutes when Rasmus Hojlund was penalised for dragging down Rodri following the intervention of the video assistant referee.
United keeper Andre Onana produced a brilliant save to deny Haaland a second from a far-post header on the stroke of half-time but the striker did not have to wait long to double City's lead from an almost identical situation four minutes after the break, Bernardo Silva delivering a perfect cross.
City keeper Ederson also excelled to save from Scott McTominay before half-time but City wrapped up this most convincing victory when Haaland set up the easiest of chances for Foden 10 minutes from time.
Manchester City have looked short of their best on occasions this season, even when spending so much time at the top of the Premier League, but the reigning champions gave an ominous display here to outclass their arch-rivals.
Haaland's hunger for goals is insatiable, with United keeper Onana doing well to deny him twice, while the manner in which they exposed the home side's flaws was brutal.
In contrast to United's shapeless efforts, City were measured, controlled and passed the ball around with the confidence of a team that knew they were up against opponents nowhere near their level.
Foden's goal gave the scoreline a far more realistic appearance as City exerted a stranglehold throughout. Further goals would not have flattered them, with the left-flank work of Jack Grealish and Bernardo Silva particularly fruitful.
The return from John Stones after injury only adds to their quality - as was proved with a performance that outlined the task facing anyone wanting to wrest the title away from them.
The sight of thousands of empty seats all around Old Trafford in the closing stages of this derby told the tale of a truly miserable and chastening day for Manchester United.
In the moments after the tribute to Charlton, the stadium was a cauldron of sounds as United fans urged their side forward - but the difference in class was swiftly clear.
United had their chances, particularly when Ederson saved from McTominay and when the struggling Marcus Rashford sent a great chance wide, but Ten Hag's side were short on ideas, creation and urgency, and were outmanoeuvred by this outstanding City team.
Hojlund may have conceded a penalty but the youngster's endeavour was admirable and Old Trafford made its feelings known with loud jeers when he was substituted late on.
United looked to have made steps forward last season, when they beat City in the Premier League at Old Trafford, but this was a worrying sight for Ten Hag, whose team looked like they were heading backwards.
-- Courtesy of BBC Sport