Sol Campbell had brought Ronaldo down 25 yards from goal and he stepped up to hit a magnificent swerving, dipping free-kick which crashed into the top right-hand corner of the net.
The shot gave Pompey keeper David James no chance and even the England goalkeeper was left with a rueful smile on his face at Ronaldo's brilliance.
"It was a marvellous hit and there is no keeper in the world that is going to save that," United boss Alex Ferguson told Sky Sports after the match.
"Without doubt that has to be the best."Ferguson meanwhile described the performance as one of United's best of the season.
"I think it was a fantastic performance by us tonight. It was one of the best this season, I think.
"I cannot believe it finished 2-0. We opened them up so many times.
"I am really pleased and it was a good team performance.
"It is nice to go back to the top of the Premier League table."