At the same time they killed off United's dream of repeating their Champions League, Premier League and FA Cup treble of 1999 after Yaya Toure struck a 53rd minute winner at Wembley.
Favourites United began positively and dominated the opening half-hour but they were then out-muscled by a City team famously dismissed by manager Sir Alex Ferguson as "noisy neighbours".
United's misery was complete when midfielder Paul Scholes was red-carded 18 minutes from time for a lunge on City defender Pablo Zabaleta and their players ended up pushing and shoving opponents and a TV cameraman in a melee after the final whistle.
In the end the night belonged to City and they will return to Wembley for the May 14 final exactly 30 years to the day since they lost to Tottenham Hotspur in a replay.
MAJOR HONOUR
City's victory, totally deserved after they recovered from a sluggish start, gives them the chance of a first major hounour for 35 years to repay the super-rich sheikh who bought the club in 2008 and has since invested a fortune in players.
The result went against the formbook with United having won their last seven matches in all competitions, including home and away wins in the Champions League quarter-finals over Chelsea in the last 10 days, while City slumped at Liverpool on Monday.
United did have to cope without in-form striker Wayne Rooney, who was suspended, but City were also wweakened by the loss of their captain and most dangerous player Carlos Tevez who pulled a hamstring in the 3-0 Premier League defeat at Anfield.
City coach Roberto Mancini was thrilled with the victory over United but warned against taking anything for granted.
"I'm very happy for our supporters because they have waited a long time for a day like this," he said. "But for us it's very important that we don't forget we have another game. We only won a semi-final today."
BERBATOV MISS
When he bought the club two-and-a-half-years ago Sheikh Mansour vowed to turn City into one of the world's top clubs, but they have yet to qualify for the Champions League or win a trophy.
For the first 25 minutes on Saturday they did not look as if they were even going to win this match.
Dimitar Berbatov spurned two chances to put United ahead 20 seconds apart in the 15th minute, seeing his first effort well saved by goalkeeper Joe Hart and then firing over the bar from two metres.
United were totally in control, but City gradually clawed their way back into the game with their midfielders pushing forward and suddenly they started creating chances with Gareth Barry, Toure, Aleksandr Kolarov and Joleon Lescott going close.
They maintained that momentum after the break when poor United defending allowed them in.
A poor clearance from Edwin van der Sar and a sloppy pass from Michael Carrick allowed Ivory Coast international Toure to escape from the United defence and he fired home through the united keeper's legs for what proved to be the winner.
"The first 15 minutes after halftime cost us the game," said Ferguson. "Slack moments. Edwin had a bad kick out and Michael Carrick couldn't hold it and it was a goal.
"From then on they were defending apart from a couple of counter-attacks. It's disappointing as we should have been ahead in the first half as we were the better team.
"The chance Dimitar missed there was a great save by the keeper but the second chance from under the bar if he'd have scored there I had a feeling whoever scored first would win the match," he added on MUTV.
