In a breathless, tension-wracked finale to one of the most dramatic and unpredictable seasons for decades, United's 1-0 win at Sunderland seemed to have sealed their 20th title but with the final whistle blown at the Stadium of Light, City produced a last twist.
In the second of five minutes of stoppage time, substitute Dzeko rose to head City level against a QPR side who had struck twice after the break despite losing former City midfielder Joey Barton to a red card for elbowing Carlos Tevez.
With just a minute of the season remaining, Aguero roared into the area to dispatch a right-foot shot past the diving Paddy Kenny for his 23rd league goal and the most precious of the 93 amassed by Roberto Mancini's City throughout the campaign.
Both United and City finished on 89 points but City's superior goal difference of eight proved decisive as the English title was settled on goals, rather than points, for only the sixth time and the first since Arsenal edged out Liverpool on the final day of the 1989-90 season.
Image: Manchester City's captain Vincent Kompany lifts the English Premier League trophy after their match against Queens Park Rangers at the Etihad Stadium, Manchester, England, Sunday May 13, 2012.