Updates with premier's remarks, police (first version posted at 20:45)
Jens Stoltenberg condemned a "cowardly" double terrorist attack that claimed at least 17 lives in Norway on Friday while police search for the motive, dpa reported.
Of the victims, seven were killed after a bomb exploded in central Oslo while 10 were killed in a shooting spree shortly afterwards at a political youth camp 10 kilomtres outside the capital.
Stoltenberg and Justice Minister Knut Storberget late Friday said the attacks were an attack on Norway's democratic system, but vowed they would not silence Norway.
The initial focus was on saving lives and aiding the victims, and Stoltenberg declined to discuss possible motives, saying those responsible must be brought to justice.
Storberget said a suspect arrested in connection with the shooting at the island of Utoya was a Norwegian man, but did not offer other details.
The suspect, who was wearing a police uniform, was seen at both scenes. It was not known if the man had acted alone.
Police Chief Sveinung Sponheim said that 10 were reported dead in the shooting on the island, but later said the toll would rise as police continued to search the island. Bomb disposal experts found explosives on the island, but Sponheim declined to offer details.
Norwegian media showed pixellated images of bodies scattered across clifftops on the island. CNN showed what it said were images of bodies in the water.
In Oslo, windows were blown out for streets around, while a mangled car was seen in television footage at the base of a 17-storey government building.
"The whole building shook. We thought it was an earthquake," said a reporter with public broadcaster NRK who was near the scene when the blast occurred.
A total of 14 people were injured, nine of them seriously, in the bomb attack.
A group calling itself "Helpers of the global jihad" claimed credit for the Oslo bombing, according to reports in the New York Times and the Swedish daily Expressen.
Stoltenberg said it happens that a group claims responsibility, "but that is not the same thing as that they have actually done it."
Police urged people to stay away from the centre of the capital and to refrain from using mobile phones to avoid overloading networks.
The offices of several media companies in the vicinity, including the newspaper VG, were also evacuated. Police later evacuated the main railway station.
The double attack was widely condemned, with President Barak Obama, German chancellor Angela Merkel and NATO's Secretary General Anders Fogh Rasmussen expressing their horror at events.