A man has been stabbed after hundreds of fans clashed outside West Ham United's stadium before their 3-1 League Cup win over London rivals Millwall.
The Metropolitan Police say the 44-year-old victim has been taken to hospital.
The police, who had been expecting trouble, say they have made two arrests so far.
There are no reports of injuries to any police officers.
Third division Millwall had led the second round tie through a Neil Harris goal until the 87th minute when Junior Stanislas equalised for the Premier League side, prompting the first of two pitch invasions by home fans.
Stanislas put West Ham in front from the penalty spot early in extra time before Zavon Hines wrapped it up for the homeside.
Police have confirmed West Ham could face an investigation into the disorder at Upton Park.