British police on Sunday arrested a 41-year-old man after a video posted on social media showed a supporter apparently making racist gestures during Saturday's Manchester derby.
"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," said Greater Manchester Police in a statement.
"A 41-year-old man has been arrested on suspicion of a racially aggravated public order and remains in custody for questioning."
City pledged to take a zero tolerance approach against supporters found to have subjected United players to racist abuse after the Premier League champions lost 2-1 to their local rivals at the Etihad Stadium.
United midfielder Fred was hit by objects thrown from the crowd as he prepared to take a corner in the second half and footage on social media appears to show a City fan mimicking a monkey at the Brazilian.
United manager Ole Gunnar Solskjaer said both Fred and Jesse Lingard had been "affected" by the alleged abuse.
The Football Association will investigate the incident and plan to talk to the clubs, referee Anthony Taylor, the police and other relevant parties.
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 alleged 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.
United star Fred said the alleged incident at the Etihad 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," the former Shakhtar Donetsk midfielder told ESPN Brazil.
"The guys showed me. He even threw a lighter and it hit me. I try not to care about that. I try to look ahead.
"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."