A group of AZ Alkmaar fans stormed the area where West Ham players' friends and family were seated in some frightening scenes at the AFAS Stadion.
David Moyes' side secured their place in the Europa Conference League final on Thursday night after beating their Dutch opponents 3-1 on aggregate, but this shocking incident at full-time marred a competitive semi-final clash.
As you can see in the footage below, a number of West Ham fans were attacked by AZ Alkmaar ultras after the full-time whistle.
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In fact, a group of furious AZ supporters hopped over the barricade and into the West Ham 'family section', where punches and missiles were thrown.
A number of stewards tried to intervene but were unable to stop the wave of AZ fans.
Several concerned West Ham players, including Declan Rice, raced over to try and calm things down, with many of their family members caught up in the melee.
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Videos from the AFAS stadium show MIchail Antonio and Flynn Downes climbing over the advertising hoardings, while others were held back.
The Guardian reporter Jacob Steinberg posted an update soon after the incident, saying: "A few AZ Alkmaar fans have disgraced themselves. Ultras attacking families of West Ham players after final whistle. West Ham deservedly through to the final."
Speaking to BT Sport after the unsavoury scenes, former Hammers midfielder Joe Cole said: "It's absurd. Ridiculous grown men, AZ Alkmaar fans attacking the fans where families were sitting, our friends and colleagues. The players tried to break it up.
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"You have to look at UEFA and AZ Alkmaar with things like that. Turning up with balaclavas throwing punches, it's just ridiculous. The players are rightfully concerned, families, friends, kids over there. Ridiculous in the modern game, pathetic.
"People talk about hooliganism in England. We don't' see things like that, we haven't for a long time. We get a reputation for it. It's other clubs in Europe, it's pathetic."
Cole added: "We have friends sitting there. In the modern game, with the cameras in these places, find out who these people are and ban them for life. We were talking about bringing our kids to the final now you second guess when you see things like that."
Topics: West Ham United, Europa Conference League