Police have made safe an explosive device found outside a busy underground station in central Oslo.
Officers evacuated late-night bars and restaurants in the Norwegian capital as bomb disposal experts rushed to the scene.
Authorities later said the device was neutralised and a suspect arrested.
Police chief Vidar Pedersen said the device, initially described as "bomb-like", was an explosive.
No injuries were reported.
Police would not give any details about the suspect or further information about the device.
Mr Pedersen said the device was found on the street just outside the Groenland underground station and police ordered people to leave bars and restaurants.
"Every restaurant was being closed," said Malin Myrvold, 23, who witnessed the scene from a fourth-storey window.
"You could see cops in heavy armour going in every store and restaurant.
"We were trying to see what was going on. The police were screaming at us to get back inside and stay where we were."
Norway was put on high alert after neighbouring Sweden suffered a truck attack in the capital Stockholm that killed four people and injured 15. The suspect in Friday's attack, a 39-year-old native of Uzbekistan, has been arrested.
It was unclear whether he was also a Swedish citizen or resident or even how long he had been in the country.
The Norwegian incident happened less than a mile from the government buildings that were damaged in a deadly bomb attack carried out by right wing extremist Anders Breivik in 2011.
That bomb and a subsequent gun massacre killed a total of 77 people.
