Celtic have urged fans in Amsterdam for tonight's Champions League clash with Ajax to be "extra vigilant".
The warning comes after reports of an attack on Hoops supporters in a city centre bar last night.
Thousands of Celtic supporters continue to flood into Holland for the Group H game at the Amsterdam ArenA.
A statement on the Celtic website said: "Celtic Football Club is urging all supporters in Amsterdam for tonight's UEFA Champions League tie with Ajax to be extra vigilant following an unprovoked attack on Celtic fans last night
"The attack in the city centre by an element of the Ajax support resulted in a number of arrests.
"Celtic are urging all supporters to be extra vigilant in the city centre and at the Amsterdam ArenA, and to only stick to the advised areas for safety reasons."
Watch highlights of Celtic's win over Ajax in Glasgow two weeks ago:
At the pre-match media conference on Tuesday afternoon, Ajax coach Frank De Boer expressed hope that the tiny percentage of fans he describes as "crazy" would not disrupt the game.
UEFA opened disciplinary proceedings against the Dutch club after some of their supporters clashed with police and stewards at Parkhead last month during a match which the home side won 2-1.
The case will be dealt with by UEFA's control and disciplinary panel on November 21.
De Boer admitted that some Ajax fans remain a concern when asked if he was confident of the game passing off trouble-free.
"I am confident in that but you never know," said the former Rangers player.
"There is always some crazy people (who) try to disturb something but hopefully it will not happen."