Celtic say they will investigate after arrests were made ahead of their Champions League group defeat to Ajax.
Violent disturbances in Amsterdam led to numerous people being detained by Dutch police and the Scottish champions released a statement to say they had already begun probing into the worrying events in Holland.
"Celtic Football Club is currently investigating events in Amsterdam last night (Wednesday)," read the statement.
"We have been informed that there are five individuals detained in Holland. We are not clear if they have any formal association with the Club, however, we will be liaising with the local authorities in Amsterdam, our own police and the British Consulate in order to fully understand events. We would stress that there is no excuse for any form of violent conduct.
"In addition, we have received numerous complaints from Celtic supporters in relation to their treatment in Amsterdam, which we will also investigate fully.
"There is no question that Celtic supporters have been subjected to a high degree of provocation. Last week, our supporters were the target of missiles being thrown from Ajax supporters at Celtic Park, a night where 300 seats were damaged within the stadium.
"This was followed up by an organised attack on a group of Celtic supporters in Amsterdam on Tuesday, the night before the match. We have also received numerous individual reports of other attacks on our fans.
"The Celtic travelling support has a magnificent reputation across Europe and our fans have visited major European cities over many years without incident. Once again the overwhelming majority of our supporters behaved impeccably."
Meanwhile Aston Villa manager Paul Lambert, who spent eight years with Celtic during his playing career, said he was surprised by news of the arrests.
He said: "I played for many years at Celtic and I never encountered any bother.
"I played in the UEFA Cup final and we took over maybe 80-90,000 to Seville and there was not one arrest while we were there.
"Knowing the club and the fans the way I do, I find it incredible (what has happened). I'm not sure they would have started anything.
"I know there was trouble in the first game at Parkhead and it might have spilled over from that.
"But if you look at Celtic over the past 30 years there has not been one bit of bother.
"So I would disagree with anybody thinking the Celtic fans would go out looking for trouble when they never do it."