Four teenagers have been injured a knife attack near a school in the town of Eskilstuna, west of Stockholm.
The case is being classified as attempted murder, but police said they had no information on how serious the injuries were. So far, no one has been arrested.
A large deployment of police was involved in an action near the Munktellarena sports facility in Eskilstuna, some 90 kilometers (56 miles) west of Stockholm, police said.
Four boys aged 15 have been taken to a nearby hospital, with three suffering stab wounds to their upper torsos, police confirmed.
READ MORE: Dramatic clip shows gun-toting driver ramming police car and speeding at 135mph
Swedish media has reported that there are several schools nearby.
Initial reports suggest violence flared between two gangs of youths in the town.
Stockholm Police said a call was received at 11.45am GMT regarding a suspected serious crime.
A spokesman added: “At the moment, several police patrols are in the vicinity of Munktellarenan, which is supposed to be the scene of the crime.
“Barriers have been set up. It must be an assault with at least one motive.
“The crime classification is attempted murder.
“At least three people are injured and they have been taken to hospital. The police press service has no details about the damage.
“At the moment, no one has been arrested in the case. Forensic technicians are going to the scene.”
An update at 1.07pm GMT said police were still at the scene gathering information and talking to witnesses.
In a second update half an hour later, the spokesman added: “The police are working to clarify what happened. Several people must be involved in a fight that resulted in the personal injuries.”