Armed police rush to centre of Oslo following explosion as force warns 'there could be more'

Armed police have now shut off the road as firefighters remain on standby to tackle any potential blazes.

A second device found at the site appeared to be a "military-style" hand grenade, officer Brian Skotnes revealed.

"We've apprehended one suspect and are intensively seeking more information as well as other people," he said.

"Our hypothesis is that it is criminals who have an agenda against other criminals, but we cannot rule our anything."

Local news has reported that a 13-year-old was apprehended by police.

However, Mr Skotnes declined to comment on the suspect's age.

Video footage showed that a number of police helicopters were also called to the scene.

Pictures, which have circulated on social media, showed armed police putting up a cordon on Pilestredet in Oslo.

Officers had told residents in the nearby area to stay away from their windows, however, Mr Skotnes said "the area was now safe".

Meanwhile, residents across Oslo have received an emergency message to their mobile phones.

Warning locals about the blast, the message said: "There has been an explosion in the area (called) Pilestredet.

"Given that there is still an explosive device in the area, the police wish that people stay away from windows."

'As of now, we are not evacuating the area.'

"We are working on the basis of a main theory that a hand grenade was planted," Oslo Police operations manager Øyvind Hammervold said.

In a statement, the force added: "Residents in the area are asked to stay away from windows. Others are asked not to visit the location.

"There are explosives at the site that have not been detonated."

Despite the site being in the immediate vicinity of a university campus, no injuries have been reported in the Norwegian capital.

The incident comes just hours after reports of drone sightings near Oslo's and Copenhagen's Airports.

Deputy Police Inspector Jakob Hansen was asked by reporters on Monday if the drones were of Russian origin, however, he said he could not confirm or deny this.