Police in Oslo found and detonated an explosive device on the street on Saturday night just outside the an underground station.
Norwegian police have set off a controlled explosion of a "bomb-like device" in central Oslo and are holding a suspect in custody in an investigation led by security police.
A Reuters reporter described a loud bang early on Sunday shortly after Oslo's bomb squad arrived with a remote-controlled robot once the area was cordoned off by police late on Saturday night.
"The noise from the blast was louder than our explosives themselves would cause," a police spokesman said, adding that further investigation was needed to find out if the device had contained explosives.
The device, about 30 cm across, had appeared to be capable of causing only a limited amount of damage. Forensics experts will examine fragments to figure out what it was.
Police across the Nordic region have been on heightened alert after a truck ploughed into a crowd in Stockholm on Friday. Four people were killed and 15 injured in what police called an apparent terror attack.
Norwegian police detained a suspect but declined to give information about his identity. Norway's police security service, PST, said in a tweet it had taken over the investigation from local police.
"We're in a very early phase of the investigation," PST spokesman Martin Bernsen said. More details were likely later on Sunday, he added.
Police took away cordons put up overnight in the Groenland area and residents resumed normal Sunday activities, with shops and cafes open. There was no sign of police at the site.
The Groenland area, a multi-ethnic neighbourhood that is home to popular bars and restaurants, several mosques, and the city's main police station. The police station is less than a kilometre away from where the device was found.
In 2011, right-wing extremist Anders Behring Breivik set off a car bomb in Oslo that killed eight people and destroyed Norway's government headquarters, before going on a shooting rampage that killed 69 people at nearby Utoeya island.