One 18-year-old male attacked at the concert in Fasta was taken to Karolinska University Hospital in a serious condition, The Local reported Sunday.

"He was hit on the back of the head with a baton and was then punched or kicked in the head," said police spokesman Gunnar Samuelsson.

The other three youths were taken to Soder hospital with no life-threatening injuries.

"We believe that the attackers are neo-Nazis," Samuelsson said. "We don't have any descriptions. Witnesses said that they had black masks on their faces."

The attacks took place at a concert organized by the youth wing of the Left Party, the Young Left, that was supposed to promote the fight against racism.

"I think it's obvious that it is Nazis who were trying to scare us into silence. But those of us who were there just felt even more angry and more motivated to continue the fight against racism," said the organization's leader, Seluah Alsaati.