A “bomb-like” device has been destroyed in a controlled explosion in central Oslo, police have said.
Officers cordoned off a large part of the Gronland area on Saturday evening, before the bomb squad detonated the suspect device.
A police spokesman said the noise was “louder” than their explosives alone would have caused. A suspect is in custody.
Police were already on alert after an attack in Sweden on Friday.
A truck ploughed into a Stockholm department store, killing four people and injuring 15 more.
It was the worst attack to hit the Nordic region since the far-right extremist Anders Brevik killed 77 people in Norway in 2011.
Swedish police confirmed on Saturday they had found an unknown device on the driver’s seat in the abandoned lorry, which they were testing.
The device in Oslo was capable of causing only limited damage, police said. The investigation has been handed over to Norway’s Police Security Service.