Manchester City says it's aware of a social-media video that appears to show a supporter making racial gestures during the Premier League match against Manchester United.
The club is also working with Greater Manchester Police to help identify the culprit.
The alleged incident occurred in the second half of City's 2-1 defeat at the Etihad Stadium, as a fan appeared to make a monkey gesture towards at least one United player.
United reported the matter to City and the referee.
"The club is also working with GMP regarding an instance of objects being thrown onto the field of play," a City statement after the game read.
"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."
United manager Ole Gunnar Solskjaer branded the alleged abuse "unacceptable".
"I've seen it on the video," he told Sky Sports. "Fred and Jesse [Lingard] in the corner taking coins from the City fans first of all, so they've got to go away.
"And then there's this not-so-intelligent fella chanting racist abuse towards my two players, and I hope City and the authorities will deal with it, because he shouldn't be watching football again. Not live, anyway.
"We have to stamp it out. We keep talking about it every single week and it's not acceptable.
"He should not be allowed back into a ground. That's how simple it is."
Manchester City boss Guardiola added on the alleged abuse: "I support the club [statement] and we are going to work to avoid it happening again."
United goalscorer Marcus Rashford said he was not aware of the missile throwing and alleged abuse until after the game.
"But it can happen and in recent times, we have been dealing with it a little bit better, but the fact it's still happening isn't good enough," he said.
"There's obviously work still to do in that area, but for us, it's about the football and trying do our best on the pitch."
English football's equality and inclusion organisation Kick It Out said it had been "inundated with reports of alleged racist abuse from a number of individuals" during the game.
Reuters.
