German police confirmed that four visiting Bayern Munich fans have been injured during an attack by suspected Olympiakos supporters ahead of the two clubs' Champions League match.
Bayern Munich said on Wednesday they have complained to UEFA after some fans needed hospital treatment following an attack by a masked gang during a Youth League match at Olympiakos.
In a statement on the club's website, Bayern said the matter is in the hands of UEFA's disciplinary and control commission after the attack in Athens during Tuesday's Under-19 match, which the Germans eventually won 4-0.
A UEFA spokesman told Bavarian broadcaster Bayerischen Rundfunk they will "start proceedings" to "ensure that this does not happen again".
According to Bayern, three German supporters needed treatment for their injuries, but were later "fortunately able to leave the hospital".
Greek police said a large group of people, wearing masks or helmets, invaded the venue where the youth team match was being held and started attacking Bayern fans.
After the incident, Olympiakos players went into the stands to ask Bayern supporters for forgiveness and the match was able to resume.
Greek club Olympiakos has condemned the attack.
"The incident shocked us," said Bayern coaches Martin Demichelis and Danny Schwarz in a statement, "our thanks go to the players, the coach and the Olympiakos officials who took care of us after these events".