West Ham players confronted a group of AZ Alkmaar fans who attacked an area in which their friends and family were watching the teams' Europa Conference League semi-final second leg.
BBC commentator Alistair Bruce-Ball, who was inside the stadium, said he saw punches being thrown by the AZ fans.
Video footage shows chaotic scenes inside the AFAS Stadion, Alkmaar, in the Netherlands, after West Ham won.