Hospitals are on standby after the attack that took place in Kongsberg, a city one hour outside Oslo, at 6 p.m.
It is not yet known how many people were injured or lost their lives in connection with the incident.
- The police say that several people were injured and killed after being hit by an arrow in Kongsberg, says Joakim Reigstad.
The perpetrator is believed to have acted alone, but residents in Kongsberg have been asked to stay at home.
A large area has been cordoned off and several police forces are in place.
- It is a small town, it is usually very quiet here in the evenings, says Joakim Reigstad.
The authorities have not yet released any details about the detainee.
He was arrested about an hour and a quarter after the very first alarm.
It is unclear whether the attack has a terrorist connection.