Norwegian mass murderer and self-proclaimed Nazi, Anders Behring Breivik, on Wednesday won his lawsuit against the state over his "inhuman" solitary confinement in prison.

The ruling was greeted with surprise by the state's lawyer and some of the survivors of Breivik's bomb and gun rampage in 2011 that killed 77 people.

"The prohibition of inhuman and degrading treatment represents a fundamental value in a democratic society," the Oslo district court said in its decision issued after hearing the case in March.

"This applies no matter what, (even) in the treatment of terrorists and killers," it said.

"The court... has concluded that the prison conditions constitute inhuman treatment."

Norway's most notorious inmate has been detained in a high-security prison unit and held apart from other inmates since the massacre, the worst peacetime atrocity in the country.

The court said Breivik's almost five-year isolation violated Article 3 of the European Convention on Human Rights.

Judge Helen Andenaes Sekulic ruled however that prison authorities had not violated his right to correspondence, guaranteed by Article 8 of the convention, by strictly controlling his communication with the outside world.

The 37-year-old, who testified in March that he was now a Nazi who had renounced violence, had asked for the restrictions on his correspondence and visits to be lifted so he could communicate with his supporters.


'Extremely dangerous'

The state had argued the restrictions were necessary because he was "extremely dangerous", and were intended to prevent his supporters from carrying out future attacks.

Wednesday's verdict means Norwegian prison authorities could be required to ease some of his conditions, although they did not immediately comment.

Breivik is serving a maximum 21-year sentence – which can be extended if he is still considered dangerous – for killing eight people in a bombing outside a government building in Oslo and then shooting dead another 69, mostly teenagers, at a Labour Youth camp on the island of Utoya on July 22, 2011.

Disguised as a police officer, he spent more than an hour hunting down the almost 600 youths trapped on the small island.

He put a bullet in the head of most of his victims, some of them up-and-coming leaders of Labour, Norway's dominant political party, which Breivik blamed for the rise of multiculturalism.


Ruling is 'absurd'

Utoya survivors had mixed reactions.

"That the court rules in Breivik's favour is a sign we have a working court system, respecting human rights even under extreme conditions," survivor Bjorn Ihler wrote on Twitter.

"What Breivik did was inhumane, which is why it's crucial to treat him humanely. He doesn't set the premise for how we treat others."

But not everyone shared his view.

"Hooray for the rule of law and all that, but this is absurd," said Viljar Hanssen, whom Breivik shot five times, including once in the head.

Breivik's lawyer Oystein Storrvik said his client would not appeal the correspondence issue.

Representatives of the state would study the verdict before deciding whether to lodge an appeal, the state's defence lawyer Marius Emberland said, adding that he was "surprised" by the ruling.

During his hearing, which was held at the high-security Skien Prison where he is being held about 130km southwest of Oslo, Breivik testified that his prison regime was having negative effects his health, citing headaches and concentration difficulties.

The state "has been trying to kill me for five years," he said, describing his isolation as "torture".

"I don't think many people would be able to survive as long as I have."

But doctors, psychiatrists and prison staff who examined him in prison testified they had seen no major change in his physical or mental state due to jail conditions.

Norway prides itself on a humane prison system aimed more at rehabilitation than punishment.

The state's lawyers had argued that his conditions fell "well within the limits of what is permitted" under the European convention, and were more comfortable than that of other prisoners.

At Skien Prison, Breivik has three cells at his disposal – one for living, one for studying and one for physical exercise. He also has a television with a DVD player, a games console, a typewriter, and books and newspapers.