Ronaldo smashed in an unstoppable 40-yard shot after six minutes and United hung on to become the first English team to win in Porto, their 3-2 aggregate victory setting up a last-four clash with Premier League rivals Arsenal.

The Portugal forward, who was jeered by the local fans with every touch, replied with an impressive individual effort for his second goal of the competition.

Ronaldo began his run from just inside the Porto half in the sixth minute and sent a rising shot past goalkeeper Helton and into the top corner.

"Unbelievable. A magnificent hit. Absolutely unbelievable," United coach Alex Ferguson said. "The goalkeeper's got no chance."

Photographs: Reuters