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Norwegian police raised the country’s security threat level to “probable” from “possible” after the device was found late on Saturday.

A number of roads were blocked off after the discovery of a potentially explosive device, police in Oslo told newspaper VG.

Spokesman Svein Arild Jørundland revealed the bomb squad were on the way after officers found a box, around 30cm square, near the bridge in Grønlandsleiret.

Norwegian authorities conducted a controlled explosion of the device, police told reporters.

There were reports of a loud bang shortly after the bomb squad arrived in the capital.

A police spokesman said: "The noise from the blast was louder than our explosives themselves would cause."

He said: "It is of such a size that we do not think it has a great damage potential, but we will have to examine it.

"We have put adequate barriers, so that public safety is safeguarded."