Manchester City 3-2 QPR: Aguero and Dzeko strike in injury time to hand Mancini's men Premier League title.
Hope seemed to be lost when the visitors took the lead despite Joey Barton's sending-off but the strikers sent fans into raptures with dramatic added-time goals to make history.
Manchester United's celebrations turned to stunned silence on Sunday as players and fans helplessly saw the Premier League title slip out of their grasp in the final few minutes of the season.
United had seemed set to defend its title in unlikely fashion when the final whistle blew in a 1-0 win at Sunderland in the final round, with Manchester City trailing 2-1 to Queens Park Rangers as that game went into added time. But City provided an even unlikelier comeback, scoring twice in the final minutes to snatch the league trophy away from its crosstown rival with a last-gasp 3-2 victory.
"It's a cruel way (to lose the title)," United manager Alex Ferguson said. "We've experienced many ups and downs in the 25 years I've been here, most of them have been great, we've won the title three times on the last day, today we nearly did it.
"I'd like to say on behalf of Manchester United, congratulations to our neighbours -- a fantastic achievement to win the Premier League."
United's players left the pitch in Sunderland with shell-shocked expressions, while the fans looked at each other in disbelief and stood with hands on heads in a bewildered silence.
The top two were level on points going into the final round, and City used its better goal difference to clinch its first title since 1968.
Sunderland manager Martin O'Neill said the result took him back to his time in charge of Scottish club Celtic when he lost the title to derby rival Rangers by a goal difference of just one in 2002.
"It's incredible really," he said. "It's just about the most harrowing thoughts you can have. You immediately start thinking about goals you could have scored during the course of the year ... it's just really unbelievable."
It was also reminiscent in a way of the 1999 Champions League final, when it was United that scored two late goals to secure a 2-1 win over Bayern Munich.
In a Premier League season full of twists and turns, United had led the standings by eight points with six games remaining at the beginning of April after City slumped to a 1-0 loss at Arsenal.
Looking to extend its record haul of league titles to 20 -- including 12 Premier League trophies under Alex Ferguson -- United seemed to have broken City's challenge.
But an unexpected loss at Wigan, followed by a shock 4-4 draw against Everton after throwing away a two-goal lead let City back into the race.
City then won the much-anticipated Manchester derby 13 days ago to edge ahead on goal difference and set up a nailbiting finale.
As the final-day showdown swung into action, United struck the first blow when defender Phil Jones sent a deep cross to the back post in the 20th minute for Wayne Rooney to score with a stooping header.
Early as it was, the goal put United top of the standings and prompted a period of intense pressure, with Sunderland goalkeeper Simon Mignolet punching away a shot from the recalled Ryan Giggs in the 28th.
Rooney almost extended the lead when he curled a free kick onto the crossbar on the half hour mark and then miskicked in front of goal after Young's shot fell to him in the 34th.
Shortly after, news that City had taken the lead against QPR filtered into the stadium, prompting the Sunderland fans to taunt the visitors with chants of "1-0 to the City boys" and suggestions that United would win nothing this season.
United's fans responded by holding up a banner with the number 19 -- the record number of titles won by the club -- while Ferguson chewed his gum pensively on the bench.
"You never know -- you're going to get certain types of reaction from the crowd," Ferguson said. "You have to put that to one side and play your own game. I thought we did quite well."
Sunderland had offered very little attacking threat in the first half and United had the better chances in the second period as Rooney had a shot saved by Mignolet in the 47th.
As time ticked on, events seemed to be favouring United as the away fans learnt of QPR's equalizer at City.
The news got even better for the travelling supporters as QPR took the lead and United almost made the win over Sunderland safe when Mignolet pushed Giggs' shot over the top in the 76th.
As the final whistle blew at the Stadium of Light, United's fans stood nervously waiting for confirmation of City's failure to beat QPR.
Incredibly, that expectation turned sour in the space of just a few minutes as City clawed its way back against QPR with two stoppage-time goals to win and wrest the title from United's grasp.
"We've had so many injuries this season and we have coped with that really well," said Ferguson, who hinted that United's future was bright despite the bitter disappointment. "We've a lot of young players. They'll be around in five, six, seven, 10 years time at Manchester United. The experience is good for them, even if it's a bad one."
The results meant the top two finished level on points for the first time since 1989 when Arsenal snatched the title from Liverpool with a 2-0 win at Anfield in the final game.
Manchester City have an incredible home record this season. Roberto Mancini’s men have won 21 of 22 games at the Etihad, scoring 51 goals in just 17 games.
