David Pizarro, the Roma midfielder, has criticised Cristiano Ronaldo for a lack of sportsmanship in Manchester United's 2-0 Champions League quarter-final first-leg win in the Italian capital last night.

Pizarro accused Ronaldo, who scored the opening goal with a header and almost made it 3-0 when he hit a post, of showing a lack of respect towards his opponents with his tricks on the pitch.

"Despite the fact that he is a great champion, he is very arrogant," Pizarro said. "He does certain spiteful things on the pitch. This is the ugliest thing for a player. In the return leg, I will have something to say to him."

Ronaldo has defended his actions after helping his team put one foot in the semi-finals. "It's nothing against my rivals," he said. "Those plays are just part of my way of interpreting football. I have always done them, even three or four years ago when I was younger.

"But there are plays that I do for the good of the team and never to ridicule my opponents. I tried to do the best for Manchester United and not to make a show. It's just a part of who I am."

Ronaldo believes Roma have improved since last year - when United eliminated them from the Champions League with a humiliating 7-1 defeat at Old Trafford at the same stage of the competition - but insists United have also also grown stronger in the last year. "I don't think we can talk about a Roma weakness but rather about the strength of Manchester United," he said. "Last season we didn't know Roma but this time, we have been perfect in defence."

Despite his team having taken a giant step towards the last four, Ronaldo prefers to remain cautious. "It would be bad to think we have done the job," he said. "We have next week's game and although a win away from home gives us confidence, we still have to seal qualification."