Atletico Madrid fans have been accused of vile racism in the stands during their Champions League defeat of Manchester United at Old Trafford.
A United fan shared video footage he filmed with the Manchester Evening News.
He said the footage clearly showed a Madrid fan directing a 'monkey gesture' towards home supporters sat next to them in the stadium's South Stand on Tuesday night.
The United fan, a 35-year-old from Bolton, also claimed Atletico supporters chanted Hitler slogans and made neo-Nazi salutes. Stewards at Old Trafford, he said, were alerted by United fans at the time and shown video footage and photographs other supporters had captured.
"It was horrible and shocking," he said. "When you see it for yourself you realise that this is real and it is worse than ever. It was just disgusting.
"You can only hope that more action will be taken. Racism is one of the biggest issues in football and it looks like it's not going away. It was horrific and kids were there.
"The Atletico fans were pretending to peel bananas and eat them in front of black United fans and fans of different ethnicities."
Greater Manchester Police, meanwhile, have confirmed officers made one arrest, followed by a second arrest at a nearby Metrolink station after the game.
Atletico Madrid won the game 1-0 to dump United out of this year's competition 2-1 on aggregate and consign the club to a fifth trophyless season. The Spanish side's players were roundly booed before kick-off as they didn't take the knee - a symbolic and united statement against racism.
The United fan, who the M.E.N. has agreed not to identify, was at the game with a friend and said racist chants and abuse happened throughout the game. "I was sat next to all the away fans," he said.
"You sort of think these days are over, bit it is clear they are not. It was really bad and to be honest, quite unbelievable. It was going on throughout the game and it seemed to be different Atletico Madrid fans.
"They were doing monkey noises and using and shouting neo-Nazi phrases. It just beggars belief.
"It was different people - they would come over to the United fans, hurl racist abuse then disappear back into the rest of the supporters. They were so many of them that were doing it. There were a lot of Nazi salutes and neo-Nazi abuse.
"It was horrible and it was shocking and a few of the United fans caught them on camera. The stewards were alerted to the situation, but nothing seemed to happen. I am not sure what the stewards did but I would imagine they would have informed the police.
"I was about to stand up and and show the stewards the footage I recorded but someone else beat me to it. People were showing the stewards the footage they had filmed."
The fan said he was more than happy to share the footage with police and any other relevant authorities.
A spokesperson for Manchester United said: "The club are aware of the incident and are now supporting the police with their investigation.
"Manchester United has zero tolerance of any form of racism or discrimination and will continue to take strong action against anyone who we identify has engaged in racist or discriminatory abuse, at our matches or online."
In a statement, GMP said: "We arrested a 31-year-old man on suspicion of a racially aggravated public order offence. He remains in custody.
"A man was also arrested at the Metrolink on suspicion of assaulting staff and possession of drugs.
"He's been released under investigation." The second arrest came after the match, the force confirmed. No further details were available.
The Manchester Evening News has contacted Atletico Madrid for comment.
After drawing 1-1 in the first away leg, Renan Lodi's 41st minute header led to United's Champions League exit at the last-16 stage. Missiles were hurled from United fans at Atletico Madrid manager Diego Simeone and the Slovenian referee as they approached the tunnel after the final whistle, with the club believed to be reviewing CCTV as a result.
