Manchester United defender Rio Ferdinand was struck with a missile thought to have been thrown from the crowd as his team's derby clash with Manchester City ended in ugly scenes.
Ferdinand was seen wiping blood from his face following the incident which took place in the aftermath of United's injury-time winner in their 3-2 victory at the Etihad Stadium.
City trailed 2-0 but fought back to equalise before Robin van Persie's late free-kick secured three points for United.
Praising the football, United manager Sir Alex Ferguson told Sky Sports: "It's just unfortunate towards the end a City fan runs on and then they throw coins at our players and that. I don't think you need that.
"The same thing happened at Chelsea which was masked really by all the other things that happened.
"But, you know, two fantastic teams today, I think, and a fantastic game of football, and it could do without that."
The violent scenes were in contrast to earlier in the match.
According to Greater Manchester Police's (GMP) Twitter feed, there had only been one arrest by 2pm of a suspected ticket tout outside the stadium.
However, during the game GMP announced that another man was arrested inside the ground on suspicion of committing a racially aggravated public order offence after officers heard racist chanting.
Earlier, the match commander Supt Craig Thompson said: "I have every confidence this match will mirror the two derbies last season and the football will be the only talking point."