Emergency services in Eskilstuna, west of the capital Stockholm, were called to the scene of the fire early in the afternoon and police are treating it as arson.

"We have eyewitnesses accounts that a person threw something into the building just before it started to burn," police officer Henrik Lagerkvist said.

Police could not confirm what had caused the fire.

A police spokesman said five people had been taken to hospital for treatment of injuries ranging from smoke inhalation to lacerations and that no suspect has been apprehended yet.

Between 15 and 20 people were in the mosque, located in the ground floor of a building in a mainly residential area, when the fire erupted.

Ibrahim Sabriya was one of those inside, and said it was a chaotic scene.

"Everyone began running towards emergency exits and windows, breaking windows to get out," he said.

"People ran into one another.

"The elderly, we threw them out of the windows, they couldn't get up to the windows so we had to throw them out."

The immigration debate has pitted the far right, which wants to cut the number of asylum seekers allowed into Sweden by 90 percent, against mainstream parties intent on preserving the Nordic country's liberal policy.

Refugee-friendly Sweden woke up to the reality of a new political landscape in early December when the anti-immigration Sweden Democrats brought down the government by refusing to back its budget proposal in parliament.

"There has been an intensification of hatred against Muslims," Sweden's Islamic Association head Omar Mustafa told public radio SR.

In January unknown perpetrators daubed black swastikas on the front door of a Stockholm mosque and in December last year neo-Nazis attacked a peaceful anti-racist protest in a Stockholm suburb, causing three people to be hospitalised.

Reuters