Our Swedish correspondent Freedom Fighter sends a brief translation, and includes this explanation:
This story is in all the Swedish press, but the police are being very secretive about it. It must have been Baptists protesting Catholic art.
Four persons were taken into custody by the national SWAT team, so something big was up.
The article from Sveriges Radio:
Sweden: Four arrested in Gothenburg on suspicion of terrorist crimes
The arrest of the four suspects at the Röda Sten Art Gallery in Gothenburg.
Röda Sten Art Gallery was cordoned off and evacuated.
The four were arrested on suspicion of preparing terrorist crimes, says Sara Kvarnström at the Swedish Security Service, to Swedish radio. Police in Sweden said that the area around the art gallery was cordoned off as a result “of a threat that involves serious danger to life and/or health and extensive destruction of property.”
Another account of the same incident may be read in Expressen (in Swedish; hat tip Steen).