Family and friends of West Ham players were attacked by rival supporters after a match in the Netherlands.
AZ Alkmaar fans broke through a fence and attacked visitors sat in a reserved section following the final whistle in the Europa Conference League semi-final.
Sky Sports reporter Gary Cotterill, who was sat close to where the chaos unfolded, said AZ Alkmaar fans who were dressed in black and had their faces covered started attacking a section of the stadium where the family members of West Ham players were.
A brawl ensued - and several West Ham players, including captain Declan Rice, tried to intervene.
Police and stewards were on the scene to prevent further confrontation and calm was restored 10 minutes later.
West Ham manager David Moyes, whose 85-year-old father was in the targeted section of the stadium, admitted that his players were angry following the incident.
Cotterill said: "West Ham players who were on the pitch celebrating after the final whistle saw what was going on and tried to come in and help their family members, others were trying to hold them back.
"But three or four of the players did jump over and go into the area and try and help their families.
"At that point stewards moved in, I must say it took some time.
"Police moved in in riot gear as well, and by then it really was chaos. You can understand the anxiousness and the concern of the West Ham players."
Cotterill said it doesn't appear any injuries were suffered as a result of the disorder.
Moyes told BT Sport after the game: "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.
"That is where the problem came and a lot of players were getting angry because they couldn't get to see if they were okay. But look, I don't know. We'll have to wait and ask the officials what really happened."
West Ham reached their first major European final since 1976 with their win over AZ Alkmaar.
They won 1-0 on Thursday to win the tie 3-1 on aggregate.
They will face Italian side Fiorentina in the final.