According to Sweden's public broadcaster SVT, the perpetrator entered Brinellskolan wielding a sword and attacked and injured several people before he was stopped. Police receiving the first calls at 2.06pm.
"We have an ongoing operation in the area and that is all we can say at the moment," says Pelle Vamstad, press officer at the police in Fagersta.
Several schools in the area have been evacuated as a result of the attack, and there are ambulances, police and police helicopters on the scene.
"We have been informed of a serious incident in Fagersta. The police are on site and are working very intensively on the incident. The government is in close contact with the police and is continuously monitoring developments," Sweden's justice minister, Gunnar Strömmer, told SVT.
According to SVT's reported on the scene, parents have arrived at the school to try and find their children, with some complaining that as a result of the ban on mobile phones in schools, they are not able to check their safety.
"I have seen police bearing weapon and there are a lot of people on the scene and a lot of worry," SVT's reporter said.