A 28-year-old man has been arrested following an alleged racist incident during Chelsea's Capital One victory over Manchester United.
Metropolitan Police confirmed the man was arrested after attending a west London police station by appointment.
Photographs taken during the fourth-round game at Stamford Bridge appeared to show a man in the crowd making 'monkey' gestures towards a player thought to be Manchester United's Danny Welbeck on 31 October.
A spokesman said: "He was arrested on suspicion of a racially aggravated Section 4a Public Order Act offence and currently remains in custody at the west London police station. Inquiries continue."
The incident occurred just a few days after a separate race row surrounded the London club.
Chelsea accused referee Mark Clattenburg of using an offensive racial term towards Jon Obi Mikel during their 3-2 Premier League defeat against Manchester United on 28 October.
Police confirm they are investigating the incident after a complaint was made by Peter Herbert, chairman of the Society of Black Lawyers.
Clattenburg has denied the allegations, which are not backed up by his assistants at the game or the fourth official.
It has since been revealed Clattenburg will not referee any games for a second week running while the inquiries are ongoing.
Mike Riley general manager of Professional Game Match Officials Limited, said: 'PGMOL was fully prepared to appoint Mark to matches this weekend.
"However, having discussed this with him, we both consider that it is in Mark's best interests that he has this week away from officiating duties while he prepares to help The FA and police with their enquiries."