More than 25 people were feared dead tonight after a bomb ripped open buildings in the heart of Norway's government and a man dressed as a police officer opened fire at an island youth camp connected to the ruling party.
At least seven people were killed in the blast in Oslo. Police confirmed that nine or 10 people were killed at youth camp shooting on Utoya island, as the peaceful nation endured the worst violence since the Second World War.
One person has been arrested after the youth camp shooting, Norway news agency NTB said.
Police say they suspect youth camp shooting and bombing in Oslo are linked.
Andre Scheie told Norwegian broadcaster NRK that he saw "very many dead by the shore" of the camp on Utoya, where the youth wing of the Labour Party was holding a summer camp for hundreds of youths.
Acting Police Chief Sveinung Sponheim said that police were still trying to get an overview of the camp shooting and could not say whether there was more than one gunman.
Aerial images broadcast by Norway's TV2 showed members of a SWAT team dressed in black arriving at the island in boats and running up the dock. Behind them, people stripped down to their underwear swam away from the island toward shore, some using flotation devices.
Sponheim said the arrested man was linked to the bombing in Oslo, the capital and the city where the Nobel Peace Prize is awarded.
A square was covered in twisted metal, shattered glass and documents expelled from surrounding buildings, which house government offices and the headquarters of some of Norway's leading newspapers.
Most of the windows in the 20-floor high-rise where Prime Minister Jens Stoltenberg and his administration work were shattered.
Stoltenberg was working at home today and was unharmed.
A spokesman for Stoltenberg's Labour Party, Per Gunnar Dahl, said he couldn't confirm that there were fatalities at Utoya, about 60 miles northwest of Oslo. The party's youth wing organises an annual summer camp on the island, and Stoltenberg had been scheduled to speak there tomorrow.
"There are at least five people who have been seriously wounded and have been transported to a local hospital," Dahl said.
He said the shooting "created a panic situation where people started to swim from the island" to escape.
Police blocked off roads leading to the lake around Utoya.
In Oslo, police said the explosion was caused by "one or more" bombs, but declined to speculate on who was behind the attack. They later sealed off the nearby offices of broadcaster TV 2 after discovering a suspicious package.
Ian Dutton, who was in a nearby hotel, said the building "shook as if it had been struck by lightning or an earthquake." He looked outside and saw "a wall of debris and smoke."
Dutton, who is from New York, said: "It wasn't any sort of a panic," he said, "It was really just people in disbelief and shock, especially in a such as safe and open country as Norway, you don't even think something like that is possible."
Public broadcaster NRK showed video of a blackened car lying on its side amid the debris. An AP reporter who was in the office of Norwegian news agency NTB said the building shook from the blast and all employees were evacuated. Down in the street, he saw one person with a bleeding leg being led away from the area.
The United States, European Union, Nato and the UK, all quickly condemned the bombing which Nato Secretary-General Anders Fogh Rasmussen deemed a "heinous act".
In Washington, US State Department spokeswoman Heide Bronke Fulton called the violence "despicable".
The attacks come as Norway grapples with a home-grown terror plot linked to al-Qaida. Two suspects are in jail awaiting charges.
Last week, a Norwegian prosecutor filed terror charges against an Iraqi-born cleric for threatening Norwegian politicians with death if he is deported from the Scandinavian country.
The indictment centred on statements that Mullah Krekar - the founder of the Kurdish Islamist group Ansar al-Islam - made to various news media, including American network NBC.