Celtic will launch their own independent inquiry into reported problems after the UEFA Cup tie in Spain against Celta Vigo.
The Parkhead club have already received a number of eye-witness accounts of the problems which occurred at the airport in Spain and on a diverted flight home last Friday.
Some fans were reportedly struck by the Spanish police as they boarded an aircraft. And a flight was diverted to Cardiff on Friday after an alleged incident involving some of the club's supporters.
Now Celtic have asked for further information from supporters who travelled to the game to help their inquiry.
A club statement said: "The tremendous victory by Celtic in the third round of the UEFA Cup has been overshadowed by certain events involving Celtic supporters.
"Eye-witness accounts reported to the club have accused the Spanish police of using unnecessary force in their attempts at crowd management at both the Celta Vigo ground and the airport.
"We have received reports of a number of injuries to Celtic supporters as a result of police-baton wielding."
Celtic today revealed they have already contacted the Foreign Office in London with a view to lodging a formal complaint with the Spanish authorities.
The club statement added: "In a separate incident, an aircraft, not chartered by Celtic or its official travel partner, was diverted from Glasgow to Cardiff en route from Santiago de Compostela.
"The reason given for the diversion was an alleged 'serious incident' on board between passengers and crew. This matter is being handled by Cardiff Police and Celtic Football Club has already indicated that it will assist the Welsh Police fully.
"We have received a number of reports which are critical of the aircrew. We wish to gather as much objective evidence as we can in order to assist the authorities, in forming their conclusions.
"Celtic supporters have a good reputation. It is of great concern to the club that these incidents might unjustly tarnish that reputation and that of the club.
"Indeed, the Vigo press were very complimentary about the behaviour of the Celtic fans."
The club say they will appoint an independent individual to lead a separate investigation into three particular incidents, specifically:
"The actions of the Spanish police and Celtic supporters at the Celta Vigo stadium; the actions of the Spanish police officers and Celtic supporters at Vigo airport; events on the diverted charter flight."
PA