The new threat level will be in place for two months. The move came after a truck ploughed into a crowd in the Swedish capital Stockholm on Friday, killing four people and injuring 15.
Authorities in Norway raised the country's security threat level following the discovery of an explosive device in central Oslo late on Saturday.
The move came after a truck ploughed into a crowd in neighbouring Sweden on Friday, killing four people and injuring 15. Swedish investigators say the main suspect was a Daesh sympathiser. No group has claimed responsibility yet.
Norway's police also arrested a 17-year-old Russian asylum seeker in connection with the explosive device which was found near a busy subway station in Norway's capital. The police set off a controlled explosion of the device Saturday night.
The risk of an attack taking place in Norway has been raised to "probable" from "possible," Marie Benedicte Bjoernland, head of the police security agency (PST), told a news conference.
The new threat level will be in place for two months, she added.
It's unclear if the teen had plans to carry out an attack with the homemade device, Bjoernland said. "PST is now working on finding his intentions and find out whether others are involved," she added.
The device, about 30 centimetres (1 foot) across, appeared to be capable of causing only a limited amount of damage.
Further investigation was needed to find out if the device had contained explosives.
Life returned to normal in the area after police took away cordons which were put up overnight.