A man has been arrested after a fan was seen apparently making monkey gestures towards Manchester United players during Saturday's derby.
Greater Manchester Police (GMP) confirmed a 41-year-old had has been arrested on suspicion of a racially aggravated public order and remains in custody for questioning.
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.
United midfielder Fred also appeared to be hit by an object hurled from the crowd as the visitors prepared to take a corner.
The Football Association will also investigate and is planning to talk to the clubs, referee Anthony Taylor, the police and other relevant parties.
It is understood 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.
Superintendent Chris Hill of GMP said: “Racism of any kind has no place in football or our society and I hope this arrest shows that we are taking this matter extremely seriously.
“We will continue to work with Manchester City and Manchester United Football clubs on this incident and will investigate any other lines of enquiries.”
Manchester United manager Ole Gunnar Solskjaer called for stern action and education in the wake of the alleged abuse.
He 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."
Manchester City said in a statement they were working with police to identify "any individuals concerned".
The statement added: "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."
Fred said the alleged incident at the Etihad showed "we are still in a backward society".
"We are all the same regardless of skin colour, hair and gender," he said.
"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."
Lingard himself branded the individual's apparent behaviour "shameful".
Commenting on Twitter post showing a video of the alleged incident, he wrote: "Not even this idiot can ruin tonight's feeling, shameful behaviour, it may be the derby but there is never ever a reason to be racist!"
Fred said the alleged incident showed "we are still in a backward society".
"On the field, I didn't see anything. I saw it only in the locker room afterwards," he said.
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.
England's players faced monkey chants and Nazi salutes in a Euro 2020 qualifier away to Bulgaria in October, while earlier on Saturday there was an incident of alleged racist abuse by a supporter towards a visiting player at the League Two match between Forest Green and Scunthorpe.