The flames reportedly spread through an air vent to other parts of the premises and raged through the night, continuing into Monday.
Local residents were ordered to stay at home. Schools and daycares have been closed and trains have been cancelled, as the main north-south trainline through Sweden passes just a few hundred metres from the fire.
No one was injured in the fire, but Polarbröd - which is the third biggest bread producer in Sweden - is an important employer in the small town of Älvsbyn in Norrbotten county. Mayor Tomas Egmark calls the fire a "tragedy".