United took advantage of two early Arsenal blunders at the Emirates Stadium to kill off the tie and book their place in the final in Rome 4-1 on aggregate.
An inspired Cristiano Ronaldo struck twice after Park Ji-sung had punched an early hole in Arsenal's defence and United's joy was only tarnished by a harsh red card for midfielder Darren Fletcher which rules him out of the final against Chelsea or Barcelona.
NEWS: Unfortunate Fletcher suspended for final
PHOTOS: Arsenal 1-3 Manchester United
United extended their record unbeaten run in the competition to 25 games after inflicting Arsenal's first defeat in Europe at their new Emirates home.
"We got the break with the first goal with the defender slipping and Park Ji-sung takes advantage of that," United manager Alex Ferguson said in a televised interview.
"The performance in terms of confidence and wanting to play our football was really terrific." United have fared badly at Arsenal in recent years, winning just one of their last 10 visits, and there was a real mood of optimism among the home fans in the opening exchanges as the visitors looked tentative.
Within 11 minutes, however, the tie was effectively over.
Arsenal manager Arsene Wenger said in his programme notes that he had been plotting for the return leg non-stop in his head since the 1-0 defeat at Old Trafford last Wednesday.
But all the Frenchman's meticulous plans went up in smoke after eight minutes as young left back Kieran Gibbs endured the worst moment of his fledgling career.
There seemed minimal danger as Ronaldo scurried to the by line and cut back a cross but the 19-year-old Gibbs lost his footing and despite desperate attempts to recover his balance, the lurking South Korean Park lifted a shot past Almunia.
BOOED LOUDEST
Ronaldo was booed loudest by the home fans when the teams were announced but the Portuguese replied with a mesmerising display of footwork and power.
Three minutes after Park's opener, the European and World Player of the Year was fouled by Robin van Persie way out on the right.
It seemed an improbable distance from goal but he only had one thought in his mind as he ran up and sent a ferocious free kick dipping inside Almunia's near post from 40 metres.
Wayne Rooney then almost made it 3-0 with a right-foot curler that Almunia tipped round the post.
The lop-sided nature of the contest continued after the break as an air of resignation hung over north London.
Ronaldo revelled in the wide open spaces down Arsenal's flanks an
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