At least three people were killed and several others were injured in a shooting incident in the Swedish city of Uppsala on Tuesday, media reports said.

In a statement, the police said they had received calls from people who heard sounds of gunshots in the city centre, news agency Reuters reported.

"Several people have been found with injuries that indicate gunfire," a police statement, available with the media, said.

The police have cordoned off a large area, and a murder investigation is underway.

The incident happened on the eve of the Walpurgis spring festival, which brings a large number of people onto the streets of Uppsala, a city located north of the capital Stockholm.

"Everything happened so fast. It just went bang, bang, bang," a witness told Swedish channel TV4.

A "major effort" is currently underway, with a police helicopter joining in the search for the shooter, police spokesman Magnus Jansson Klarin told TV4.

Train services had been initially stopped in the area to stop the perpetrator from fleeing the crime scene, Klarin said. They have now resumed.

A "brutal act of violence" has happened, Justice Minister Gunnar Strömmer told TV4.

The identity of the victims and the shooter are yet to be ascertained.

The incident comes amid an increasing concern over the number of shootings and gang attacks in recent years in Sweden.

The government has said it wants to tighten the Swedish gun laws.