Aaron Cresswell of West Ham United Photo: PHOTOSPORT
AZ Alkmaar say their Europa Conference League semi-final against West Ham is "a night to reflect on with shame" after a section of the stadium where away fans were sitting was attacked.
Several West Ham players confronted the group of fans that had targeted an area containing their friends and family.
AZ said in a statement of apology: "Unfortunately, we cannot use the word 'supporters' for these people."
The club said they will "evaluate" the incident.
"While everyone hoped for a historic European match, it turned into a pitch black evening due to the events occurring at the referee's final whistle," AZ's statement added.
"It turned into a night to reflect on with shame. Not because of the football game played, but because of the behaviour of some visitors.
"What happened is beyond all bounds. The club again sincerely apologises to West Ham and the thousands of well minded AZ supporters who have also been inconvenienced by the misconduct.
"AZ is a civilised club where sportsmanship and norms and values are paramount. The club will do everything possible, together with the authorities involved, to identify these persons and to take appropriate measures."
West Ham's first-team coach Kevin Nolan has urged Uefa to issue tough punishments.
"If this was England, there would be lots of sanctions put on us. I hope they put loads of sanctions on the Dutch," Nolan said on 5 Live's Friday Football Social.
"I am disgusted that they would come and go for family which is what they did."
'There were stewards but no police'
Watch: Chaotic scenes as fans clash inside the AFAS Stadion, Alkmaar
A West Ham supporter sat in the area attacked by AZ fans said there were no police nearby and stewards could not cope, describing the incident at the AFAS Stadion as "scary".
"There were stewards there but no police to stop them," Russell Raphael told the BBC News Channel.
"More than 30 of them in balaclavas somehow breached their area and were running towards us.
"Some very brave West Ham fans, about 10 rows to our right, were doing their best to stop them, the stewards were doing their best.
"The players instead of celebrating with the main away support panicked and saw their families being threatened, so they very understandably ran to get involved.
"These [fans in] balaclavas, hooded, all dressed in black, it is a scary thing and we were very lucky not to be physically involved."
West Ham players confront AZ fans after 'awful scenes'
Latest West Ham United news, analysis and fan views
Michail Antonio and Said Benrahma were among the players who tried to intervene, while the 87-year-old father of West Ham manager David Moyes was reportedly sat in the stands in the Netherlands.
It is understood the families of the players were unharmed.
West Ham won the game 1-0 for a 3-1 aggregate victory.
"The security going in was fine. You expect to be herded like cattle in these away games, that's the way it is," added Raphael, who watched the match with his son, Aidan.
"They kept us back for a long time after the game and there was a lot of police going out. I didn't feel in danger around the ground. In the ground it was just stewards who were clearly unable to cope with what was going on.
"It must have taken 10 minutes for the police to arrive."
European football's governing body Uefa will review reports of the incidents before deciding on any action.
Dutch police said they were investigating footage of the incident but no arrests had been made.
'There was nothing we could do' - Moyes
"I was disappointed because I felt like it took the attention off the team and what we did," said Moyes on Friday.
"There was nothing we could do, or that West Ham as a club could do. Like anyone, if you saw your family members in trouble then you'd go and help out and that was the situation.
"Unfortunately it's taken up a lot of the news and time. But we're not letting it take away from the fact we reached the final, which is a brilliant achievement."
AZ Alkmaar manager Pascal Jansen said he was "a little bit ashamed" that the incident had happened in their stadium, while Pantelis Hatzidiakos was one of several AZ players who condemned the violence.
"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," the Greece defender told Dutch television.
"I don't even call them supporters. Just stay home if you have such intentions."
- BBC
