Manchester City have apologised to Rio Ferdinand after the Manchester United defender appeared to be hit by a coin as he celebrated his side's third goal in their 3-2 victory at the Etihad Stadium.
Ferdinand sustained a cut just above the eye in the incident, which is now being investigated by Greater Manchester Police.
"We apologise to Rio," said a Manchester City spokesperson. "We condemn the actions of the individual concerned. We are reviewing the CCTV with the police and our normal security. We will support the police with any investigation and hopefully identify the person as soon as possible."
Ferdinand was able to complete the game and afterwards on Twitter said: "Whoever threw that coin, what a shot! Can't believe it was a copper 2pcould have at least been a Â£1 coin!"
Sir Alex Ferguson adopted a more serious tone, and the United boss said: "It was disappointing when the supporter ran on the pitch and Rio got hit in the eye by a coin. The game didn't deserve that."
It was a sad postscript to a thrilling afternoon in which United had established a lead through Wayne Rooney's double before City hit back. Once Pablo Zabaleta had levelled after Yaya Toure had halved the deficit, it seemed like the hosts would go on to win, only for Robin van Persie to seal the spoils for United, who move six points clear at the top.
"We should have been out of sight with the chances we had," said Ferguson. "But you have to give credit to City. They keep going and they score late goals and they have done it against us again - fortunately we got the last one."
It left Roberto Mancini to reflect on another dismal performance from Mario Balotelli. The Italian was preferred to Carlos Tevez but after an ineffective opening period, showed no sign of improvement, triggering a stark warning from his manager.
"I love Mario but it is important for him to start to think about his job," said Mancini. "He has everything to play well but he can't continue to play like (this). We wanted more from him.
"I have seen players in my life with fantastic quality. But in the end, they did nothing. I don't want Mario to finish like these players. It would be bad for him."