Swedish police have arrested a teen suspect after three youths aged from 15 to 20 were killed in a shooting at a hair salon, authorities said on Wednesday, with rising concern over gang violence in the Scandinavian nation.

Tuesday's shooting took place in broad daylight in the university city of Uppsala, a day before it hosted the Valborg spring festival, which draws more than 100,000 people for bonfires and celebrations in the streets, many of them students.

"One person has been arrested, suspected of murder," police commander Erik Akerlund told reporters, with prosecutors saying the suspect was 16.

The teen had slipped away from a facility for troubled teens two weeks ago, according to prosecutors and local media.

Swedish media reported that at least one of the dead had connections to a criminal gang, though police would not confirm those reports.

"That is something we are obviously looking into, but we don't want to commit ourselves to only that possibility," Uppsala police spokesman Stefan Larsson said.

Uppsala, about 70 kilometres north of Stockholm, is the home base for Sweden's two most notorious gangs, Rumba and Foxtrot.

Their leaders, Ismail Abdo and Rawa Majid, are both now believed to be orchestrating operations from abroad.

Swedish Prime Minister Ulf Kristersson called the shooting "an extremely violent act."

"It almost looks like an execution," he told the news agency TT.

Sweden has struggled for years to rein in shootings and bombings between rival gangs.

Akerlund said several people "considered of interest in the investigation" had been brought in for questioning.

He said police had obtained camera footage from the scene and "secured a lot of evidence."

Prosecutor Andreas Nygren said the suspect was arrested at his home around two hours after the shooting, and that "more than 100 interrogations" had been held as of Wednesday morning. (AFP)