Norwegian police have arrested three men suspected of planning attacks and having links to al-Qaeda and people under investigation in the United States and Britain.
The arrests were brought forward by fears details of the investigation would appear in public.
"We were afraid evidence would be destroyed, because we knew that an international media organisation was about to publish details of the case. That made it urgent to make the arrests," said Janne Kristiansen, head of the Police Security Service (PST).
The police chief earlier told a news conference the three were arrested for "planning a terrorist strike".
"We believe this group has had links to people abroad who can be linked to al-Qaeda, and to people who are involved in investigations in other countries, among others the United States and Britain," she said.
Norwegian police said two of the men were arrested in the Oslo region while a third one was arrested in Germany in cooperation with German police.
"The PST has today ... apprehended three individuals suspected of having prepared terrorist acts," it said in a statement.
One of the men was a 39-year-old Norwegian citizen of Uighur origin, another was a 37-year-old Iraqi citizen, and the third was a 31-year-old Uzbek citizen. The Iraqi and the Uzbek both had permanent residency in Norway.
Ms Kristiansen said Norwegian police had collaborated with several police services abroad, but did not specify which. She did not say which country was the target of the planned attack.
"The threat of terrorism in Norway is low and it is still low," she told the news conference.
Reuters