Manchester United's Cristiano Ronaldo celebrates scoring a goal against Arsenal during the UEFA Champions League second leg semi-final match at the Emirates stadium in London, on Tuesday night.

Ronaldo scored twice as the defending champions beat Arsenal 3-1 in the second leg semi-final for a 4-1 aggregate.

Park Ji-Sung (centre) scores past Arsenal goalkeeper Manuel Almunia to give Manchester United the lead in the 11th minute.

There seemed minimal danger as Ronaldo scurried to the by line and cut back a low cross but the 19-year-old Kieran Gibbs lost his footing and despite his desperate attempts to recover his balance, the lurking South Korean Park lifted a shot past Almunia.

Arsenal goalkeeper Almunia is beaten by a free kick from Ronaldo (not in picture).

Three minutes after Park's opener, European and World Player of the Year Ronaldo 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.

Arsenal captain Cesc Fabregas (left) and Anderson of Manchester United fight for the ball.

It extended United's record unbeaten run in the competition to 25 games and inflicted Arsenal's first defeat in Europe at their new Emirates home.

Ronaldo is jubilant after scoring the third goal.

Park and Ronaldo played a one two before Park sent Wayne Rooney marauding deep into Arsenal territory to slide a pass across the area for Ronaldo to clip a shot past Almunia.

"I thought tonight they couldn't handle him," Manchester United manager Alex Ferguson said of Ronaldo.

"Yes, we had a midfield that worked its socks off and had good shape and good cover, but we needed someone to give them real problems and Ronaldo in that form...well he's fantastic."

Manchester United midfielder Darren Fletcher (left) is shown the red card by referee Roberto Rosetti as team mate Rio Ferdinand (centre) reacts.

The 25-year-old Scotland captain, who had an outstanding match until his 75th minute sending-off, was adjudged by Italian referee Roberto Rosetti to have brought down Fabregas and denied him a scoring opportunity.

Rosetti, though, showed him an immediate red card and awarded Arsenal a penalty which produced little more than a consolation goal for the dispirited home team who were 3-0 down on the night and 4-0 behind on aggregate.