Baton-wielding Swedish police officers have been filmed clashing with fans inside the Celtic section of AIK stadium.
Footage shows the officers laying into supporters on Thursday night after the fans travelled to the Stockholm stadium for the Europa League second-leg match.
The ugly scenes began just before half-time although it is not yet clear what started the incident - however Celtic chiefs have since claimed they believe fans of AIK's rivals Hammarby .
One witness said: "Five minutes before half time the police move along to where these guys were standing, just to my left, pointing people out.
"As soon as HT whistle blew they barged in, started grabbing guys & utterly smashing them with batons."
Some fans speculated that the officers entered the section to pick out fans who had set off pyrotechnics at the start of the game.
Celtic have been fined on various occasions by UEFA after their fans let off fireworks and flares during European matches.
The most recent incident took place in February 2017 when fireworks were let off during the game against Manchester City at the Etihad Stadium.
Celtic could now end up in the dock again with UEFA due to the flares set off prior to kick-off and the clashes with police just before half time.
Both matters are set to be included in the Match Delegate’s report which will be sent to European football's governing body.
Some Celtic fans later took to Twitter to slam their fellow supporters who let of pyrotechnics before the game.
With UEFA likely poised to hammer the club again, the fans believe such behaviour will only damage the club.
One said: "After the past week or so, Celtic fans were warned how tough UEFA are going to be in regards to misconduct.
"Setting off pyros and clashing with Police will almost certainly see Celtic hammered by UEFA. Idiots."
Another added: "Celtic fans who set flares off attracted the attention of the police.
"Blame them for the brutality."
And a third said: "Flares let off before by Celtic fans, they are idiots for doing that, sorry but they are, but later on there's video of the police being very heavy handed and laying in to some fans."
The trouble in Sweden comes a week after and fans inside Celtic Park.
Celtic eventually won Thursday night's match against AIK 4-1, having already triumphed 2-0 in Glasgow last Thursday.
