Police in Oslo have found a "bomb-like" device and are holding a suspect in custody.
"We are investigating and we have detained one suspect," the country's police force said in a tweet.
A large part of the city centre was cordoned off as police investigated further.
The device was later detonated in a controlled explosion.
Norwegian news site NRK said police spotted a suspicious man in the area, questioned him him and then found the device.
NRK quoted a police spokesman as saying that "it would appear that this is an object that does not have huge potential for injuries."
"I can not confirm [it is a bomb], but it is an object that is about 30 by 30 centimetres, that is to say that there is potential for damage in it, but it is of such a nature that we want the bomb squad to examine it,"police spokesman Svein Arild Jørundland told NRK.
The incident happened less than a mile from the government buildings that were damaged in a deadly bomb attack carried out by right wing extremist Anders Breivik in 2011. That bomb and a subsequent gun massacre killed a total of 77 people.
Eivind Flognfeldt, the owner of a nearby pub, told NRK that his staff had been asked to evacuate the premises.
Norway has tightened security after the Stockholm truck attack in which four people were killed and 15 injured, with its police officers armed "until further notice."
Those planning to cross the Norwegian border with Sweden are advised to bring their passports, as Swedish police have introduced tighter border controls following Friday's attack in Stockholm.
Swedish police decided to enhance the country's border controls following the suspected terror attack in Stockholm on Friday. This would likely affect travellers planning to cross the border to Sweden, Norwegian police said.
Vi har kontroll på stedet, og avventer nå til vi har fått foretatt nødvendige undersøkelser.Vi har kontroll på en person med status mistenkt— Politiet i Oslo OPS (@oslopolitiops) April 8, 2017
"The police can reinforce the control at all border checkpoints, but I don't know anything about where and when they are going to do it," police spokesperson Mattias Lindholm told NRK.
"My general advice is that everyone planning to travel to and from Sweden should bring their passport," he said.
Sweden's borders will remain reinforced for ten days, Swedish police said.