West Ham supporters came under attack from AZ Alkmaar fans at the end of their Europa Conference League semi-final second-leg at the AZ Stadium on Thursday night.
The Hammers defeated AZ Alkmaar 1-0 and 3-1 on aggregate, meaning they will take on Italian side Fiorentina in the cup final in Prague on 7 June.
After the full-time whistle, a group of about 50 AZ fans forced their way into the West Ham section of the main stand, where they began to attack Hammers supporters.
Order was restored 10 minutes later. No arrests were made.