The fire started in an apartment building in Mölndal outside of Gothenburg early Monday morning.

Neither police nor emergency services could say what caused the fire, although the head of the tenant-owners’ association told the TT news agency that the fire started when someone threw a mattress on a balcony after an electric heating pad began to overheat.

“The mattress smoldered for several hours on the balcony and then caught fire. The fire spread up the eaves and into the attic. It was there about 30 metres were burned away,” said the head of the association, adding that he estimated the damage would come to tens of millions of kronor.

Lars Ango from the Västra Götaland police said the entire block was evacuated.

“We’re talking about lots and lots of people,” he told TT.

Emergency services received a call about the fire around 3.45am.

More than 40 firefighters from four different departments were called to fight the flames, emergency services reported.

By 8am, the fire had more or less been extinguished.

“Now all that remains is the final work of putting out the fire,” said emergency service operator Urban Samuelsson to TT.

Some of the hundreds of people evacuated may be able to return home later in the morning.