Police said a 43-year-old man was in a stable condition in hospital after being stabbed mid-evening in nearby Priory Road.
Another man was found in Rancliffe Road with head injuries and another said he had been assaulted at a pub in Green Street.
There was trouble within the ground during the Carling Cup match after West Ham fans invaded the pitch on three occasions.
AT THE SCENE
Ben Jacobs, BBC, Upton Park
It all started before kick off, outside Upton Park Underground, where hundreds of rival fans broke through a strong police barrier, throwing bottles and even bricks - one of which I personally saw almost hit a pregnant woman.
Police riot team personnel - 500, possibly 1,000 - tried to dissipate the situation. The scuffles filtered to some side roads where one man was stabbed.
These were nasty scenes; not one or two mindless individuals. There were 100 or 200 mindless individuals.
Afterwards, Millwall fans were kept in the stadium for 20 minutes or half an hour. That, though, didn't stop hundreds more clashing after the game.
It was the first time the rival London sides had met since April 2005 and West Ham won the game 3-1 after extra time.
By 2300 BST several hundred West Ham fans had congregated outside the stadium where they chanted and bombarded police with beer cans and bottles.
One pub reportedly had its windows broken by bricks during running battles across several streets up to half-a-mile from the stadium.
Eyewitnesses also told of hooligans trying to rip out bollards and lighting "small fires".
A 29-year-old owner of a kebab shop said he closed his premises in nearby Green Street for two hours.
He said: "All hell broke loose - it was very frightening.
"There were West Ham fans on one side and Millwall on the other. They were being kept apart by the police.
"Bottles and bricks were being thrown from the back and some were hitting West Ham fans at the front.
"There were loads of people with bloodied faces."
One resident, who watched from his window as the trouble flared, said hundreds of fans gathered on the corner and threw bricks and cans at rivals.
"Riot and mounted police arrived to disperse the fans but they were outnumbered," he said.
Tony, a steward at the game, told BBC Radio 5 live they were powerless to stop the violence.
He said: "When you have 2,900 Millwall fans it is really intimidating.
"There is not a lot you can do - you have to hope the police presence can help you out."
Harry Hallamn, 14, travelled to the game with his brother and parents from their home in Horsham, Sussex.
He said the Tube driver told passengers he was unable to stop at West Ham station due to "police investigations".
Harry said: "I was really scared, especially for my mum and dad, as I knew the Millwall fans would be waiting for us at the stations.
"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.
"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."
Minister for Sport Gerry Sutcliffe said: "The scenes last night were a disgrace to football.
"I completely back the FA's call for any person identified as involved to be banned for life."
He added: "We have made great progress in the past 20 years in tackling football hooliganism in this country and will not tolerate any return to the dark days of the 70s and 80s."
In a statement, West Ham Football Club said it would fully investigate this "deplorable violence" and would take the strongest action against anyone involved.
A Football Association spokesman said anyone found to have taken part in the violence would be banned from football for life.
He said: "We absolutely condemn the disorder.
"We will be working with all parties, the police and clubs, to establish the facts surrounding these events."
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