Police in Sweden say four people have been arrested on terrorism-related charges in Goteborg, the country's second-largest city.
The four were arrested on the eve of the 9/11 anniversary after officials learned of a threat against an arts centre in the coastal city in western Sweden.
Police cordoned off the Roda Sten culture and arts arena and the facility was evacuated around midnight.
About 400 people were attending a contemporary art gala when police ordered everyone to leave, said Mia Christersdotter Norman, the head of Roda Sten.
Police searched the building but didn't find anything related to the threat, she said.
Officials wouldn't say whether the suspected plot is linked to the 10-year anniversary of the Sept. 11 terrorist attacks in the United States.
It was not the first such case where a group of civilians has been threatened with violence or become the target of it in recent years in the Nordic region.
In December, suicide bomber Taimour Abdulwahab set off a bomb in downtown Stockholm among Christmas shoppers, injuring two people and killing himself.
Norwegian mass killer Anders Behring Breivik admitted to killing 77 people on July 22 — eight in a bombing in Oslo outside government buildings and 69 gunned down on nearby Utoeya Island at a political youth camp.