Bolton Wanderers have made an official complaint about "overzealous and disproportionate" treatment of their supporters by Spanish police.
Bolton fans were baton charged by Spanish police in Madrid
The club contacted European football's governing body Uefa after the 0-0 draw at Atletico Madrid in the Uefa Cup.
Club chairman Phil Gartside said he would take the issue to the "highest level" to get an explanation.
Bolton said there were "a number of assaults and unprovoked baton charges" and about 17 fans were injured.
British police also criticised Spanish officers' "heavy-handed" tactics.
James Kirk, one of an estimated 3,500 Bolton fans in Madrid, said: "We came out of the stadium and just as we came down loads of police with shields and batons rushed back just where we'd come from.
"They were battering people to the ground - it was absolutely brutal and appalling. I've been to Spain many, many times but I tell you I don't want to go again."
Another fan said: "I've been all over the country watching Bolton and the police in Spain behaved like animals. I have never seen anything like it."
Mr Gartside said: "I have great sympathy for any of our supporters who have been injured by what is clearly an overreaction by the Spanish police.
"Bolton Wanderers Football Club will be pursuing the matter at the highest level and seeking an explanation from the Spanish authorities."
The football club is liaising with Greater Manchester Police (GMP) and the British Embassy to try to establish what happened.
Ch Supt Dave Lea, of GMP, said the problems started when police baton charged hundreds of Madrid fans out of the stadium.
"That incident has probably set the tone for the policing of the whole match and unfortunately the Madrid police have a reputation for not taking dissent very well," he said.
The police chief said a number of sporadic incidents followed which resulted in police being "very heavy-handed" with the Bolton fans, and he blamed senior officers for the problems.
"They didn't have enough control over their junior officers who were far too quick to use their batons rather than talking to the fans who would comply with their instructions.
"Lessons need to be learned and I will make sure that my observations will be captured in a full report for the Uefa security investigation that I know is going to be forthcoming," he added.
The result of the match at the Vicente Calderon stadium meant Bolton won the tie 1-0 on aggregate to reach the last-16 stage of the competition.
Before their Uefa Cup match against Red Star Belgrade in December, Bolton fans were detained at a hotel in the Serbian capital.
Local police acted amid security fears after trouble involving Red Star supporters the previous weekend.
Last April, a number of Tottenham fans were injured at their Uefa Cup tie in Seville in violent clashes with Spanish riot police.
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