Police added: "We're now awaiting further necessary investigations".
The device was left on busy street outside Groenland underground station in the centre of Oslo and a large area was cordoned off.
Enquiries are continuing at the scene, but detectives believe the explosive was capable of causing only a limited amount of damage.
Norwegian police discovered a "bomb-like device" in central Oslo Saturday night.
The noise suggested the device might have contained explosives, Svein Arild Jorundland, police commander, said.
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Signe Aaling, chief prosecutor with the PST security service, said on Sunday that the youth had been detained on suspicion of handling explosives.
Norway has made a decision to raise the country's threat level after a homemade explosive device was found in central Oslo and a teen asylum seeker from Russian Federation was charged, the police security service (PST) said on Sunday.
Police were already on alert after an attack in Sweden on Friday.
The area is a multi-ethnic neighbourhood, and it is home to popular bars and restaurants, several mosques and the city's main police station. The suspect in the attack, a 39-year-old native of Uzbekistan, was a failed asylum-seeker who eluded police efforts to deport him. The bomb squad used a robot to blow up the small object and no one was hurt.
On July 22, 2011, Anders Behring Breivik detonated a 950-kilogram auto bomb outside the office of the then Norwegian prime minster and current North Atlantic Treaty Organisation chief Jens Stoltenberg, killing eight people. That bomb and a subsequent gun massacre killed a total of 77 people.