Norwegian police said on Tuesday they were investigating a blast on a street in central Oslo and later conducted a controlled detonation of a second explosive device found on the site, adding that a suspect had been apprehended for questioning.

There were no reports of injuries at the location, in the immediate vicinity of a university campus and some 500 metres (546 yards) from the royal palace and the Israeli embassy.

In a rare step, authorities sent an emergency message to Oslo residents' mobile phones, warning them about the blast.

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.

"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."

The area was now safe, he added.

A 13-year-old was apprehended by police, TV2 and daily Aftenposten reported, citing unnamed sources.

Skotnes declined to comment on the suspect's age.

The incident comes a day after reports of drone sightings near Oslo's and Copenhagen's airports.

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