A 41-year-old man has been arrested in connection with the allegations of racist abuse by a Manchester City supporter during Saturday's derby at the Etihad Stadium.
A video circulating on social media appeared to show a fan aiming monkey chants towards Manchester United players during the Red Devils' 2-1 victory.
City swiftly issued a statement saying the club was working with Greater Manchester Police, and Greater Manchester Police announced on Sunday AM local time that the man had been arrested on suspicion of a racially-aggravated public order offence.
A GMP statement read: "A man has been arrested after a video posted on social media showed a supporter making racist gestures at the Manchester City v Manchester United match at the Etihad Stadium yesterday.
"At around 6.55pm on Saturday, December 7, 2019 police were called to a report of a supporter making alleged racist gestures and sounds towards players during the Manchester City and Manchester United derby.
"A 41-year-old man has been arrested on suspicion of a racially aggravated public order and remains in custody for questioning."
Superintendent Chris Hill of the City of Manchester Division said the force was taking the matter "extremely seriously".
The Football Association will also investigate and is planning to talk to the clubs, referee Anthony Taylor, the police and other relevant parties.
United reported the incident to City and Taylor, with more than one of their players feeling they had abuse of a racist nature aimed at them.
There were also reports of missiles being thrown as tempers boiled over in the stands, with Fred appearing to be hit by an object hurled from the crowd as the visitors prepared to take a corner.
United boss Ole Gunnar Solskjaer said: "Fred and Jesse (Lingard) were in the corner, taking a corner, and I've seen the video, heard from the boys.
"We keep talking about it every bloody week. But he's been caught on camera. He should, in my opinion, never be allowed into a football ground again and I've seen the video. Unacceptable."
City's statement read: "Manchester City FC are aware of a video circulating on social media which appears to show a supporter making racial gestures during the second half of the match against Manchester United this evening.
"Officials from the club are working with Greater Manchester Police in order to help them identify any individuals concerned and assist with their enquiries.
"The club are also working with GMP regarding an instance of objects being thrown onto the field of play.
"The club operates a zero tolerance policy regarding discrimination of any kind, and anyone found guilty of racial abuse will be banned from the club for life."
Australian Associated Press