A 13-year-old boy is reportedly among three people arrested after an explosion rang out in the centre of Oslo, writes dailymail.co.uk.

Residents were urged to stay indoors and away from their windows as police confirmed that an undetonated explosive remained at the site following the initial blast.

Pictures from the scene on Pilestredet in Oslo, Norway, show armed police have shut off the road and the surrounding area, with firefighters understood to be on standby.

Video footage also shows that several police helicopters were called to the incident, first reported by police at 7.43pm tonight.

The second device found at the site appeared to be a "military-style" hand grenade, the police officer in charge at the scene, Brian Skotnes, told reporters at the scene.

"We`ve apprehended one suspect and are intensively seeking more information as well as other people", - Skotnes said. "Our hypothesis is that it is criminals who have an agenda against other criminals, but we cannot rule out anything".

A 13-year-old has been arrested by police, TV2 are reporting, citing unnamed sources. Skotnes declined to comment on the suspect`s age. Police have confirmed two others have been arrested in connection with the explosion as the force continues to investigate.