Dutch club AZ Alkmaar apologised for a "pitch black evening"after sections of their own support attacked family and friends of the West Ham United squad celebrating their success in Thursday's Europa Conference League semifinal.
Home supporters broke through a fence and attacked the West Ham contingent seated in a reserved section behind their own team's dugout at the AZ Stadium.
A brawl ensued in which West Ham players, including captain Declan Rice, attempted to intervene, with several running to the aid of relatives.
Police and stewards were quickly on the scene to prevent further confrontation and calm was restored after 10 minutes.
"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 said in a statement on Friday.
"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. Unfortunately, we cannot use the word 'supporters' for these people.
"What happened is beyond all bounds. The club again sincerely apologises to West Ham United and the thousands of well minded AZ supporters who have also been inconvenienced by the misconduct."
The club said that together with the police, Public Prosecution Service, and Alkmaar's municipality they would review footage to evaluate "exactly what happened, how it could have happened and what needs to be improved".
AZ added that appropriate consequences would follow for those responsible.
Dutch police on Friday told the ANP news agency they had not yet made any arrests but will review images to see if there was any criminal liability that might warrant an arrest.
There were also incidents outside the stadium, including around Alkmaar's train station, police said.
The coach and players of AZ condemned sections of their support who participated in the attack.
"It's annoying, it doesn't belong in our stadium. Preferably in no stadium," said AZ coach Pascal Jansen, whose side lost 1-0 on Thursday as West Ham completed a 3-1 aggregate victory to reach the final against Fiorentina in Prague on June 7.
"I feel shame. You have to control your emotions, even on a night with a defeat."
West Ham manager David Moyes said the incident had overshadowed the Premier League club's achievement.
"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," Moyes told reporters on Friday.
"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."
There was a similar incident in London last week when AZ supporters, including the family of coach Jansen, were accosted by West Ham fans after they celebrated the goal scored by the Dutch club in the first half of the first leg.