Several "military-style" hand grenades were discovered in the vicinity of the Israeli embassy and the Royal Palace

Norwegian police are investigating a blast that shook central Oslo on Tuesday evening, prompting emergency alerts to residents and the discovery of additional explosive devices, according to local media reports.

The first explosion occurred on Pilestredet street around 8:45pm local time, near a university campus and about 500 meters from both the royal palace and the Israeli embassy.

Authorities sent an emergency mobile alert to Oslo residents warning of the explosion and instructing people in the area to stay away from windows after a second device, described as a "military-style" hand grenade, was found at the scene.

"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," the message read.

Police cordoned off a large section of the district, disrupted tram services, and diverted road traffic before carrying out a controlled detonation.

The grenade was reportedly left on a tram line, Norwegian broadcaster NRK said. Fire and rescue crews were also deployed to assist police. Oslo's tram network in the area was shut down while security forces worked to secure the scene.

There were no reports of injuries. Incident commander Brian Skotnes told reporters that one suspect had been taken into custody for questioning. Local media, including Aftenposten, reported that the arrested individual was a 13-year-old boy, though police declined to confirm the suspect's age.

"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."