West Ham players confronted AZ Alkmaar fans who 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.
West Ham manager David Moyes said his players were “angry”.
“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,” he said.
“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.
“We’ll have to wait and ask the officials what really happened. I can only tell you that players were involved because that was the family area.”
West Ham reached their first major European final since 1976 by defeating their Dutch opponents 1-0 on the night and 3-1 on aggregate.
Moyes added: “What I don’t want to do in any way is blight our night. It wasn’t West Ham supporters looking for trouble.”
Former Hammers midfielder Joe Cole, who was part of the BT Sport team at the match, said what happened was “absurd”.
“Grown men attacking the West Ham fans,” he said. “Players were trying to get involved to break it up.
“AZ Alkmaar fans turning up wearing balaclavas throwing punches is ridiculous. There were families.”
Former West Ham forward Carlton Cole said there was “no need” for the trouble.
“West Ham have come here, done their job and not disrespected them in any shape or form,” he said on BT Sport.
‘I can see punches being thrown’
Bruce-Ball, who was commentating for BBC Radio 5 Live in the AFAS Stadion, described the scenes as “unpleasant”.
“Some fans clad in dark coats and hoods up came racing down in front of the lower tier of the stand to voice their dissatisfaction, and there is trouble to our right-hand side,” he told listeners.
“The concern here is I think the West Ham family members and friends are in that section. Those are very, very unpleasant scenes.
“The West Ham players are being held back. I can see in the distance punches being thrown.”