The mosque, in Strömsund, is located in the cellar of an apartment block.
“We received the alarm at 2:40am and the fire was extinguished soon afterwards. We do not yet know how serious the damage was, but there was at least smoke damage,” said fire brigade duty commander Bengt Stadin on Friday morning.
No people were injured in the blaze. Witnesses say they saw youngsters running from the scene.
“The rescue coordinator said there were signs that the fire was started deliberately and we are taking this very seriously. We will be treating this as a hate crime,” Stadin said.