A man was slashed in the face during a fight which police believe involved fans of English football teams Millwall and Everton before their FA Cup match on Saturday, while the FA confirmed that it is investigating a separate incident of racist chanting heard during the game.
London's Metropolitan Police said the man, who is in his 20s, was taken to a hospital and that his injuries are not life-threatening.
Video has circulated on social media showing a large group of men, purportedly Everton and Millwall supporters, fighting in south London and Detective Inspector Darren Young, of the Metropolitan Police, said the force is aware of the footage, which has "quite rightly elicited shock and disgust," and is looking to identify those involved.
On Sunday, the Football Association also confirmed it is aware of reports of racist chanting and will be opening an investigation.
"The FA can confirm that we are aware of a video posted on social media after The Emirates FA Cup match between Millwall and Everton," an FA spokesperson said. "Discriminatory behaviour and chanting has no place in football and we will be investigating the matter."
A statement from Millwall read: "Millwall Football Club is aware of, and extremely disappointed by, a video circulating online displaying alleged racist chanting during Saturday's victory over Everton. The club will, as is commonplace following such incidents, work with all relevant authorities during investigations into the matter and look to identify individuals involved. Anyone identified and guilty of such abuse will be banned from The Den for life.
"The club would like to place on record its gratitude to, and pride in, the vast majority of its supporters who gave their team outstanding backing throughout the match and who deserve to enjoy a memorable victory. They should be able to do so without being tarnished by behaviour they would find abhorrent.
"Millwall Football Club has a long and proud record of anti-discrimination work and on top of this investigation will continue to work tirelessly, alongside the authorities, to eradicate all forms from the game.
Anti-discrimination charity Kick It Out also released a statement, saying: "The racist chanting heard from some Millwall supporters during yesterday's match is disgusting and typical of mass discriminatory chants that continue to go unpunished."
"We reiterate our message: racial hatred -- and all forms of discrimination -- have no place in society, especially not in 2019. There is no reason why a footballing environment should be the exception to this rule.
"Kick It Out wants to see the relevant authorities take swift, firm and decisive action against the perpetrators."
Everton lost the fourth round game 3-2 and their fans were held back in the stadium after the final whistle on safety grounds after the fight.
Information from the Associated Press was used in this report.