Greater Manchester Police said it was questioning a 41-year-old man who was arrested after being seen on camera at City’s Etihad Stadium appearing to racially abuse United players by imitating a monkey during the away side’s 2-1 victory. He remained in custody on Sunday evening.
A Football Association spokesperson had previously announced it would also be investigating the matter.
United reported the incident to City and referee Anthony Taylor, with more than one of their players feeling they had suffered racist abuse.
Video footage also showed missiles being thrown, with the United midfielder Fred appearing to be hit on the back of the head 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.”
Fred said the alleged incident at the Etihad showed “we are still in a backward society”.
He told ESPN Brazil: “Unfortunately, this is happening in some stadiums. It happened here, it happened in Ukraine with some friends. It’s sad, but we have to keep our heads up and forget about that. We can’t give them any attention because that’s all they want. I spoke to the referee after the match, they will do something about it and that’s all.
“We are all the same regardless of skin colour, hair and gender. We came from the same place and we all go to the same place when it’s all said and done. Thank God I have a lot of friends here in the locker room who hugged me, like Lingard. I don’t want to think about it. I just want to move on.”
Manchester City said in a statement they were aware of the video and were working with police to identify any individuals who may be involved.
It said: “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.”
The issue of racism in English football has hit the headlines again virtually a year to the day since City’s Raheem Sterling was subjected to racially abusive language and threatening and aggressive behaviour at Stamford Bridge that led to a Chelsea supporter being permanently banned.
In October, England’s players faced monkey chants and Nazi salutes in a Euro 2020 qualifier away to Bulgaria, while earlier on Saturday a visiting player allegedly experienced racist abuse by a supporter at the League Two match between Forest Green and Scunthorpe.