Five people were dead after a Flixbus veered to the right of the busy A9 motorway, local media reports
Police said a man in his 30s was taken into custody on suspicion of murder following the stabbing, which happened close to where Centre Party leader Annie Loof was giving a speech.
Police also said they had no indications of further threats to the festival, and declined to comment on motives or political affiliations of the suspect.
Daily Aftonbladet said, citing sources, that the man had links to a neo-Nazi organisation.
The week-long political festival is an annual event that draws crowds of thousands as politicians hold speeches and mingle with voters and tourists.