At least 13 people have been arrested and three had to go to hospital after violence broke out at a football match between West Ham United and Millwall.
Fights started near West Ham's stadium, Upton Park in east London, a long time before the match started, and were still going on five hours later.
About 200 riot police and 20 officers on horseback were at the scene.
During the match fans ran onto the pitch three times, causing play to be stopped while police removed them.
A investigation is now going ahead, and police are looking at video footage of the fighting to see who was involved.
Reports say shops shut near the fighting, and one pub had its windows broken by bricks.
"A disgrace to football"
Harry Hallamn, 14, travelled to the game with his brother and parents on the London Underground.
He said: "As we got off the Tube there was a large number of Millwall fans waiting on the platform trying to start fights but there was a large police presence.
Fans ran onto the pitch during the game
"After the match we saw West Ham fans walking straight towards the away end of the stadium so people were looking for trouble.
"It was a disgrace to football."
"No place in our game"
The Football Association announced that anyone involved in the violence should be banned for life saying: "They have no place in our game."
West Ham agree, with the club's chief executive saying anyone responsible for a disturbance would get a life ban from the ground.
The fighting has shocked Hammers manager Gianfranco Zola, who said the people who caused the trouble were not fans of football.
The match was won by West Ham 3-1 in extra-time.