Police in the Norwegian capital maintained a tight cordon around the scene of the blast in the city centre.
Police have a suspect in custody and have said he has posted right-wing statements on the internet, but Prime Minister Jens Stoltenberg cautioned that it was too soon to know the motive for the attacks.
The gunman has been named by local media as Anders Behring Breivik. His Facebook page appeared to have been blocked by late evening. Earlier, it had listed interests including bodybuilding, conservative politics and freemasonry.
Norwegian media said he had set up a Twitter account a few days ago and posted a single message on July 17 saying: “One person with a belief is equal to the force of 100,000 who have only interests,” a misquote of the philosopher John Stuart Mill.
‘Paradise turned to hell’
Speaking about the shooting spree on Utoya island Jens Stoltenberg said “a youth paradise had been turned into hell”.
“Many of those who lost their lives were persons I know. I know the young people and I know their parents,” he told a news conference.
“And what hurts more is that this place where I have been every summer since 1979, and where I have experienced joy, commitment and security, has been hit by brutal violence; a youth paradise has been transformed into a hell.”
“What happened at Utoya is a national tragedy,” he said of the small wooded holiday island that was hosting the annual camp for the Labour Party’s youth wing.
“Not since World War II has our country seen a greater crime.”
The prime minister said he did not want to speculate on the motives of the attacks, but added: “Compared to other countries I wouldn’t say we have a big problem with right-wing extremists in Norway.
But we have had some groups, we have followed them before, and our police is aware that there are some right-wing groups”.
“There have been some groups of that kind in Norway, but again I will not speculate. We will await the investigation from the police before we say anything about the case.”