A man was slashed across the face during a mass brawl before Millwall and Everton’s FA Cup fourth round clash.
The Metropolitan Police said it believed the violence involved groups of rival fans.
Trouble flared at 16:42 GMT on Saturday in the Hawkstone Road area of Southwark, near Millwall’s home ground, The Den.
The Met said a “large group of males” was fighting and a man in his 20s suffered a slash wound to the face.
He was taken to a south London hospital with injuries that are not life-threatening.
Videos of the brawl have been watched more than a million times online.
Det Insp Darren Young, of the South Central Command Unit, said: “The behaviour of those involved in this incident is nothing short of disgraceful and those involved can be certain we will be working to identify them.
“We are aware of the video circulating online, which has quite rightly elicited shock and disgust.”
Millwall won the match 3-2.
The FA said it was not investigating the violence because it happened outside the football ground.
However, it said it was investigating reports of “a discriminatory song” being sung by Millwall fans at the match.
Millwall FC said they were “extremely disappointed” by a video which allegedly showed some fans singing racist chants.
The club said they would “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 said.