Police in the German city of Frankfurt are investigating a video that appears to show a fan being punched and kicked while being detained by a group of Euro 2024 security personnel at Portugal's game with Slovenia.

Tournament organiser UEFA said Wednesday it was "aware of an incident which took place between stewards and a fan from the Portugal vs. Slovenia match and condemns any violent behavior."

"The incident in the video is now the subject of a police investigation. UEFA is not in a position to comment further until the investigation has been completed."

The video, which was published Tuesday in Portuguese media, purports to show a group of tournament security staff detaining two people in a concrete tunnel leading toward the field at Monday's game in Frankfurt.

While one person is detained face-down on the floor, a group of people in vests marked "Steward" surround another person against the wall and one of them appears to punch the person three times in the head. That person is later shown on the ground as other security personnel appear to punch and kick them.

"We are aware of this and are already investigating," the Frankfurt police posted on X, formerly Twitter, on Wednesday morning in response to a user who asked about the incident.

Football Supporters Europe, a group which brings together fan groups from across the continent and works with UEFA, said in a statement: "Anyone found to be responsible has no place in a stadium and must be held to account for their actions."

Portugal's win over Slovenia earned the team a place in the quarterfinals. The identities of the people involved and the circumstances leading up to the events shown could not immediately be verified.

Security at Euro 2024 has been under scrutiny over repeated lapses, which included fans entering the field at a Portugal game to seek selfies from star player Cristiano Ronaldo. UEFA pledged to tighten security after that incident.

UEFA has also faced questions over a German YouTuber who said he made it onto the field at the opening ceremony disguised in a mascot costume he bought online.

A man was also seen climbing on the stadium roof with a large rucksack when Germany played Denmark on Saturday. Local police said they detained that man and believed that he was a would-be photographer who didn't have any intent to cause harm.