Family and friends of West Ham United players, celebrating their success in Thursday's Europa Conference League semifinal in the Netherlands, were attacked by AZ Alkmaar supporters at the final whistle.
AZ supporters broke through a fence and attacked the West Ham contingent seated in a reserved section at the AZ Stadium. A brawl ensued in which West Ham players, including captain Declan Rice, attempted to intervene.
Police and stewards were quickly on the scene to prevent further confrontation and calm was restored 10 minutes later.
"We'll need to wait for the dust to settle to see what it is but the biggest problem is that is the area where the players have all their families in, friends and family," West Ham manager David Moyes told BT Sport.
"That is where the problem came and a lot of players were getting angry because they couldn't get to see if they were OK. But look, I don't know. We'll have to wait and ask the officials what really happened."
West Ham did not provide further comment when reached by ESPN.
"It's annoying, it doesn't belong in our stadium. Preferably in no stadium," AZ coach Pascal Jansen said. "I feel shame. You have to control your emotions, even on a night with a defeat."
Defender Pantelis Hatzidiakos was among several AZ players who spoke out against the violence in postmatch interviews.
"I think it's sad what happened. My family was up there. I have been in contact with them and my girlfriend said they were shaking," he told Dutch television.
"Such a beautiful evening, such a great atmosphere, I really enjoyed it until the final whistle. What happened after that I find very sad and a pity."
There was a similar incident in London last week when AZ supporters, including the family of coach Pascal Jansen, were accosted by West Ham fans after they celebrated the goal scored by the Dutch club in the first half of the first leg.
West Ham came back to win last Thursday's match 2-1 and then scored a last gasp winner in the second leg of the semifinal for a 3-1 aggregate triumph.