The motive for the attack, which took place in several places in the city center of Kongsberg around 18:30 (3:30 am AEDT), has not yet been known.
Police said the suspect was taken to a police station in the nearby town of Drammen, but gave no other details about the man.
“There is no active search for other people,” Mr Aas said.
The TV2 station reported that the man also has a knife or other weapons.
The injured were taken to hospital. However, police did not say how many people were injured or given details of their condition.
An arrow sticking in a wall
Police urged the public to stay home and several neighborhoods were cordoned off, with television footage showing ambulances and armed police in the area.
A helicopter and bomber crew were also sent to the site.
The website of public broadcaster NRK published an image sent by a witness of a black arrow sticking out of a wall.
Police were expected to hold another press conference at 22:00 (7:00 AEDT).
Norway has traditionally been a peaceful nation but has suffered from far-right attacks.
Right-wing extremist Anders Behring Breivik carried out twin attacks that killed 77 people on 22 July 2011.
Breivik first set off a bomb in the capital Oslo next to the building that housed the prime minister’s office, then fired at a summer camp for left-wing youths on the island of Utoya.
In August 2019, self-proclaimed neo-Nazi Philip Manshaus opened fire on a mosque in the vicinity of Oslo before being overwhelmed by fans, and no one was seriously injured.
However, he had earlier shot dead his stepsister, who had been adopted by China, in what prosecutors called a “racist act”.
Several planned jihadist attacks were also thwarted by security services.