Racist chanting from a section of Millwall fans during the club's FA Cup victory over Everton is "disgusting" and must be punished, says anti-discrimination charity Kick It Out.
Millwall said they will "ban for life" anyone indentified using racist abuse.
Supporters could be heard using a racist term that is derogatory to the Pakistani community during Saturday's 3-2 win at The Den.
The Football Association confirmed it is aware and is investigating.
Kick It Out said such behaviour has "no place in society", while the Championship club added they are "extremely disappointed" about the incident.
"Millwall 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," a statement read.
In a separate incident, a man was slashed across the face during a mass brawl before the game that Metropolitan Police believes involved groups of rival fans.
A Kick It Out statement regarding the chanting said: "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."
Michael Avery, from Millwall Supporters' Club, said the chants and fighting were the actions of a "mindless few" and something the Championship club have been working hard to eradicate.
"We won family club of the year in 2017, we are universally recognised for the work we do with family supporters and young supporters and this kind of thing does not help the club at all. There really is no place for it in football," Avery told BBC Radio 5 live.
"It's just a mindless few. It's not right at all."