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
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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.
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.
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.
Ronaldo revelled in the wide open spaces down Arsenal's flanks an
