A man has been stabbed and violent battles have broken out between rival football fans in "large-scale" crowd trouble, police said.Hundreds of fans are involved in the disorder which police said was caused by people without tickets to the West Ham versus Millwall game who had planned to start trouble.
A spokesman for Metropolitan Police said more officers were being rushed to the scene outside Upton Park, in east London.
"There is large-scale trouble involving hundreds of fans which we are dealing with at the moment," he added.
Officers have made two arrests, one for disorder and the second for breach of a banning order.
Police said the stabbing, in Priory Road close to the stadium, is connected to the fighting and the man is being treated by paramedics.
On the Millwall Independent Supporters Association website fans were discussing the reports of violence.
One fan, calling himself Uwe said: "Already this is turning into the biggest **** storm for a while. Hopefully it won't **** the club too much. Met have made their own bed by not giving us the tickets and then saying trouble was caused by 'ticketless supporters'."
Inside Upton Park pockets of violence also broke out. Some supporters made it on to the pitch.
The rival fans taunted one another as news of the fighting outside filtered through. The game finished 1-1 meaning an extra 30 minutes would be played
West Ham now lead 3-1.