A 59-year-old man was arrested following an alleged racially aggravated public order offence at Saturday's FA Cup match between Liverpool and Manchester United.
Merseyside Police said the man from North Wales had been taken to a police station, where he would be questioned by officers, Sky News reported.
Police made the arrest after studying TV footage following the posting of an image on Twitter, which showed a man making a monkey gesture in the crowd, according to several reports.
The vile pose – posted on Twitter – was condemned by Liverpool fans, many of whom had brought anti-racist banners along to the match.
It came amid tension as the two clubs met for the first time since Liverpool striker Luis Suarez was given an eight-match ban for racially abusing defender Evra.
The 'monkey’ incident came after 23 minutes of the highly charged match, which Liverpool won 2-1 to reach the fifth round.
Captain Steven Gerrard hit a shot over the bar as Evra closed him down before the camera panned to the fans where the thug, among Liverpool supporters but not wearing club colours, makes his gesture. Police immediately launched an investigation.
A total of 17 people were ejected from Anfield either before or during Liverpool's 2-1 win over United, the rivals' first meeting since the unsavoury race row between Luis Suarez and Patrice Evra erupted in a Premier League match in October.
The unknown racist was widely condemned from fans of all sides on Twitter last night.
Bob Stone posted: “If a Liverpool fan was making monkey gestures, it should be condemned by everyone.
“Not acceptable and casts an ugly shadow over a great day.”
Louise Wain agreed: “Liverpool fan making monkey gestures...disgraceful human being.”
Arsenal fan Andy Wood said: “Hope Liverpool FC find the moron who was making monkey gestures and ban him for life. One presumes he never celebrates a Glen Johnson goal?”
Fans on Liverpool FC’s website also criticised the man.
A fan calling himself Liver-01 said: “His actions should equal a life ban and nothing less.
“Especially after how much the pressure is on our club in recent times and how the reputation is being affected.”
TheRealSheepHead said: “There were always going to be some insignificant amoeba’s bringing the names of both clubs into disrepute.
“That intellectual midget making the monkey gesture can prepare for his life-long banning from Anfield.”
Before the game both managers had called for calm.
Sir Alex Ferguson wrote to all Man Utd fans travelling to Anfield calling for them to behave.
The United boss asked for the fans to offer “positive, witty and loud” support.
And in the match programme Liverpool’s Kenny Dalglish said: “Clearly, passions always run high when the clubs play against each other but as far as we are concerned this is a football match and not about any other issue. We don’t need any sideshows.”
Evra was booed loudly every time he touched the ball during the Cup tie, which Liverpool won 2–1.
When Kenny Dalglish was asked what he thought of Evra's every touch being booed, the Liverpool manager responded: "Are you winding me up?" He added: "Why would I be disappointed for Evra? I can't believe you have asked that question before anything else. Have you ever played football? I used to get booed.
"The media has played its part this week in trying to maintain dignity and respect on the pitch and to concentrate on the game. The behaviour of both sets of players on the pitch was a credit to the game. There was banter between the two sets of fans but it was friendly. There may not have been a lot of respect, but both sets of supporters were brilliant. I don't think there was anything there that was untoward."