A West Ham United supporter was taken to hospital after a group of supporters were attacked by German fans.
The Hammers are set to take on Eintracht Frankfurt in the Europa League semi-final second leg in Germany on Thursday.
German police confirmed that a set of Eintracht fans attacked West Ham supporters, which resulted in the incident.
Around 15 to 20 fans were wearing face coverings and stormed into a pub and attacked visiting supporters.
"The injured guest, a 34-year-old man from Britain, was taken to hospital for medical treatment," Frankfurt police said in a statement.
West Ham took roughly 2,600 fans to the game, but more are expected in the city to watch the game in bars and pubs.