Police in Germany say more than 30 people were arrested overnight after incidents involving Eintracht Frankfurt and West Ham supporters.
The sides meet on Thursday in the second leg of their Europa League semi-final, which Eintracht lead 2-1 on aggregate.
Frankfurt police confirmed the arrests were because of fan skirmishes.
They said two West Ham fans were knocked unconscious and treated in hospital after an attack by home fans.
Officers broke up a gathering of an estimated 800 West Ham fans around the city’s main railway station where “supporters of both teams sought confrontation”.
There had already been trouble on Tuesday night when West Ham supporters were attacked by a group, said to be 20-strong, who were wearing Eintracht Frankfurt colours.
