Norway's intelligence services have raised the national terror threat level, after a homemade bomb was found in the centre of the capital 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".
The teenager moved to Norway in 2010 and had applied for asylum, Reuters news agency reports.
Benedicte Bjornland, head of the Police Security Service, said: "It's not clear whether the 17-year-old young man intended to commit a terrorist attack."
Recent attacks in France, Germany, the UK, Russia and Sweden were "having a contagious effect in Norway" and could lead to copycat attacks, Ms Bjornland added.
The raised threat level will remain in force for two months.
The explosive device in Gronland was capable of causing only limited damage, police said.
Police were already on alert after an attack in Sweden on Friday.
A truck ploughed into a Stockholm department store, killing four people and injuring 15 more.
It was the worst attack to hit Scandinavia since the far-right extremist Anders Brevik killed 77 people in Norway in 2011.