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.
A video of the brawl has 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.”
A crime scene remains in place. There have been no arrests.
The Met added that officers “worked to separate the groups with support from the Mounted Branch and Dog Support Unit”.
Millwall won the match 3-2.