Bolton have met police and Uefa officials in Spain to discuss the treatment of their fans at Thursday's 0-0 Uefa Cup draw with Atletico Madrid. The Premier League club says supporters were subjected to "overzealous and disproportionate" policing before, during and after the match.
They allege fans faced a number of assaults and unprovoked baton charges. Chairman Phil Gartside said he would take the issue to the "highest level" to get an explanation.
Uefa, European football's governing body, is waiting to receive the report of match delegate Odd Flattum and the security officer before deciding whether to take action against the La Liga club.
Spanish police said the problems, which saw 17 away supporters injured, were "just the usual at any football game" and refused to respond to Bolton's allegations.
Greater Manchester Police (GMP), whose chief superintendent David Lea was in the Vicente Calderon stadium, insists Bolton fans were not at fault and has pledged to help the club compile a report to Uefa.
A club statement read: "The directors of Bolton are upset and disturbed at the apparent over-zealous and disproportionate actions of the Spanish police towards our supporters.
"The club's fans were subjected to a number of assaults and unprovoked baton charges before, during, and after the game, in addition to the aggressive use of police horses to manoeuvre fans towards the stadium.
"Bolton supporters have an impeccable reputation and an excellent record of good behaviour when travelling abroad to European matches, and the club believes the actions of the Madrid police are entirely unacceptable."