The Norway gunman who killed 77 people in twin attacks in July asked an Oslo court on Monday to release him immediately, saying his massacre was a "preventive attack against state traitors."

"I do not accept imprisonment. I demand to be immediately released," Anders Behring Breivik, a 32-year-old right-wing extremist, told the Oslo court that was convened for a hearing on his detention.

Hollow laughter erupted in the rows where survivors and families of the victims were seated, when Behring Breivik twice demanded his immediate release.

The July 22 massacre was "a preventive attack against state traitors" committed to "defend the ethnic Norwegian population," he told the court.

Behring Breivik, who has claimed to be on a crusade against multi-culturalism and the "Muslim invasion" of Europe, set off a car bomb outside government buildings in Oslo, killing eight people.

He then went to Utoeya island, some 40 kilometres (25 miles) northwest of Oslo, and, dressed as a police officer, spent more than an hour methodically shooting and killing another 69 people, mainly teens, attending a summer camp hosted by the ruling Labour Party's youth wing.