Norway police raise terror level after Oslo 'explosive device' found

Police in Norway have raised their national threat level following the discovery of an improvised explosive device in central Oslo late on Saturday.

The Police Security Service (PST) evacuated a busy nightlife street in the city centre after finding the explosive device.

Bars and restaurants were cleared before bomb disposal experts safely detonated the small-scale device, while a suspect has been taken into custody.

Police said a 17-year-old Russian citizen is in custody and is a suspect in the case.

PST chief Benedicte Bjoernland told a new conference: "The risk of an attack in the coming months has been raised to 'probable' from 'possible'."

Bomb disposal experts were called to the scene. Credit: AP

A witness described frantic scenes as police ordered the public to retreat to safety after the discovery on a street just outside the central Groenland underground station.

"Every restaurant was being closed," Malin Myrvold, 23, told Reuters after seeing the operation from a fourth-floor window.

"You could see cops in heavy armour going in every store and restaurant," she said.

"We were trying to see what was going on. The police were screaming at us to get back inside and stay where we were."

The discovery in Oslo comes with the city on increased alert following Friday's attack in neighbouring Sweden. Credit: AP

Police earlier said on their official Twitter account they believed the device "only (had) potential for causing limited damage".

The discovery came a day after Norwegian police confirmed armed officers would patrol the country's biggest cities following Friday's suspected terror attack in the capital of Scandinavian neighbours Sweden.

Police said they will carry weapons "until further notice" in the wake of the lorry crash in Stockholm, which killed four people and wounded 15.