Around 10 people have been killed in a shooting at an adult education centre, Swedish police said, the deadliest attack to take place in Sweden on what the prime minister said was a "painful day" for the country.

The gunman is believed to be among those killed and a search is continuing at the school for other possible victims, the local police chief told a press conference. The perpetrator's motive was not immediately known.

"We know that 10 or so people have been killed here today. The reason that we can't be more exact currently is that the extent of the incident is so large," local police chief Roberto Eid Forest told reporters.

Forest said police believed the gunman had acted alone and that they did not currently suspect terrorism as a motive, though he cautioned that much remained unknown. He said the suspected gunman had not previously been known to police.

"We have a big crime scene, we have to complete the searches we are conducting in the school. There are a number of investigative steps we are taking: a profile of the perpetrator, witness interviews ... Obviously, it's a significant amount of work," he said.

Police said they had opened an investigation into murder, arson and an aggravated weapons offence and that the search for possible further victims had continued into the evening.

The shooting took place in Orebro, some 200 kilometres west of Stockholm, at the Risbergska school for adults who did not complete their formal education or failed to get the grades to continue to higher education. It is located on a campus that also houses schools for children.

"It is a very painful day for the whole of Sweden," Prime Minister Ulf Kristersson said on X.

"My thoughts are (also) with all of those whose normal school day was turned into fear. Being locked up in a classroom fearing for your life is a nightmare no one should have to experience." (Reuters)