Fourteen people received minor injuries in the fire which was started around noon on Wednesday at a building in central Stockholm housing the embassies of Portugal, Tunisia, Argentina and Belgium.
Portugal's Foreign Minister Augusto Santos Silva told Portuguese media that the blaze appeared to have been an “isolated act” started by a “disturbed” visitor to his country's embassy and not terror-related.
The minister said that the man, in his fifties, had turned up at the embassy and spoken incoherently in Spanish and English, apparently referring to alleged problems of human trafficking in Portugal.
“He demanded to speak to the person in charge of the embassy's consular section, which was granted, but before the conversation had even begun he started the fire and ran off,” said Santos Silva.
The man fled the scene but was later apprehended by Swedish police.
Swedish police said they had briefly questioned the man on Wednesday, but would not yet comment on a possible motive. Forensic teams were set to examine the scene of the fire on Friday.