The "abhorrent behaviour" of rival football fans at the Millwall versus Everton FA Cup match on Saturday was "some of the most shocking football violence seen for some time", Scotland Yard said.
Deputy assistant commissioner Matt Twist said officers were analysing video footage to identify dozens of people involved in the disorder and were also looking into allegations of racist chanting during the match.
Around 200 officers were on duty to deal with the match, but as the situation escalated more specially trained officers had to be sent to the game.
Millwall's dramatic 3-2 win was marred by the crowd trouble outside the ground with videos emerging appearing to show some home fans singing racist chants during the match.
A spokesman for the Football Association has said the governing body was aware of the incident and was investigating.