Norway's intelligence services have raised the national terror threat level, after a homemade bomb was found in the centre of Oslo.
The risk of an attack was now "probable" rather than "possible", the security services said.
On Saturday, police cordoned off a large part of Gronland in central Oslo, and carried out a controlled detonation on an explosive device. A 17-year-old Russian citizen has been arrested. His lawyer told local media that he denied any wrongdoing and that his actions were "a boy's prank".
"It's not clear whether the 17-year-old young man intended to commit a terrorist attack," the head of the Police Security Service, Benedicte Bjornland, said
Recent attacks in France, Germany, the UK, Russia and Sweden were "having a contagious effect in Norway" and could lead to copycat attacks, Bjornland added.
The raised threat level will remain in force for two months, BBC reported.
