West Ham players confronted AZ Alkmaar fans after they appeared to attack an area in which friends and family were watching the teams' Europa Conference League semi-final second leg.
Michail Antonio and Said Benrahma were among those to intervene in the clashes just moments after the Premier League team had reached the final.
BBC commentator Alistair Bruce-Ball, who was inside the stadium, said he saw punches being thrown by the AZ fans.
"These are awful scenes," he said.
More to follow.