Dutch fans stormed through a gate dividing the two sets of supporters and tried to get a small block of 100 West Ham fans - including families - who were still celebrating Fornals’ 94th-minute strike. Missiles were seen being thrown with the players attempting to intervene.
West Ham's stars, including captain Rice, attempted to cool tensions and pleaded with fans to calm down. Rice furiously gestured for the fighting to stop and was seen throwing an object back off the pitch and into the crowd after losing his cool in one moment.
Footage of the incident showed police using batons in a desperate attempt to restore order. West Ham fans were then seen celebrating with their own supporters as the tension started to simmer, with club legend Mark Noble seen in tears.
"A sensational evening. One of the greatest in the club's history. There's a bit of trouble breaking out I have to tell you, among some of the rival fans, which is sad to report,” said BT Sport commentator Ian Darke.
“Some have decided they want to get involved in a few fisticuffs. But that's a side issue, really. Declan Rice is getting involved and saying, 'Let's stop this. Let’s everybody just calm down a little bit’. It's just taking the edge off West Ham's celebrations here.
“It's all proved just a little bit too much for some of the Dutch fans and they've come over to try and pick a fight with the West Ham supporters. Some of the West Ham players are clambered down there. You can see Aaron Cresswell. This is all getting a bit out of hand."
Co-commentator Robbie Savage added: "The West Ham players need to get out of there. It's not a pleasant scene at all. Players getting involved - just stay out of the way."
Darke continued: "The police are getting involved as well. Those Alkmaar fans who came across are now being herded and a few batons are being struck as well. Now West Ham can celebrate with their supporters. Unfortunate, but the real story is of a splendid performance from West Ham."
“The players, rightfully so, concerned: families over there, their friends, maybe kids over there. Absolutely ridiculous in the modern game, and the men that turn up and do it: completely and utterly pathetic.”
AZ are likely to face a strong punishment from UEFA. West Ham boss David Moyes said: "We'll need to wait until the dust settles to see what it is. I think the biggest problem is that it was the area where all the players had their friends and families.
“That's where the problem came. A lot of the players were getting angry because they couldn't get to them to see if they were OK. I don't know. We need to wait and see the officials what really happened."
Fornals struck a stunning goal four minutes into injury time to put West Ham into their first European final in 47 years. The 1-0 victory was enough to seal a 3-1 aggregate triumph.
