Police said on Monday that they were working to track down the person responsible for throwing a coin at Manchester United's Rio Ferdinand during Sunday's Manchester derby.
Ferdinand was left with blood streaming down his face from a cut above his eye after being struck by a coin as he celebrated Robin van Persie's injury-time winner in United's 3-2 victory at Manchester City.
Greater Manchester Police said that they had made 13 arrests and charged nine people over offences that occurred before, during and after the match.
"To have just 13 arrests for a crowd of this size and a match of this proportion is a testament to the policing operation we put in place," said Chief Inspector Steve Howard.
"Despite fierce rivalry and high tension, there was no major disorder. However, we will continue to investigate the coin-throwing incident and are determined to work with the club to bring the perpetrator to justice."
Among the people charged were two men accused of entering the field of play during the game.
City goalkeeper Joe Hart had to restrain one young fan from getting at Ferdinand after the coin-throwing incident, prompting the United defender to thank his one-time England colleague on Twitter.
A 30-year-old man was also charged with a "racially aggravated public order" offence.
All the people charged are due to appear before Manchester City Magistrates' Court on 4 January 2013.
United's victory took them six points clear of defending champions City at the top of the Premier League table.