The ruling, which took many by surprise, found that the killer had been subjected to strip searches, had been woken up hourly by guards for long periods and that the authorities had done little to alleviate the impact of his isolation.
Breivik killed eight people in a bomb attack in Oslo in July 2011 before attacking a youth meeting of the Labour Party on an island to the northwest of the capital, killing 69 people.
Breivik took Norwegian authorities to court in March, accusing them of exposing him to inhuman, degrading treatment or punishment in breach of the European Convention on Human Rights.
He protested his isolation from other inmates and from outsiders who are not professionals.
"The prohibition of inhuman and degrading treatment represents a fundamental value in a democratic society. This applies no matter what - also in the treatment of terrorists and killers," judge Helen Andenaes Sekulic said in her ruling.
The verdict said the Norwegian state had broken Article 3 of the convention, pointing to the fact that Breivik is spending 22 to 23 hours a day alone in his cell.
"It`s a completely locked world with very little human contact," it said, adding that there had been no attempt to ease the security "even though Breivik has behaved in an exemplary manner during his time in prison".
His isolation is "an inhuman treatment" in the meaning of the European convention, it said, noting that all his visits, except for his mother who died in 2013, are from professionals, and only through a glass wall.
The wall must be seen as a "completely exaggerated security measure," said the verdict.
The ruling, however, said the Norwegian state had not violated Breivik`s right to a private and family life. It said that strict censorship of his letters was "in line with the law".
In March, the case raised dismay, and some laughter, among Norwegians taken aback by Breivik`s complaints of cold coffee and microwaved meals he said were "worse than waterboarding".
Breivik`s lawyer said prison authorities must ease the isolation of his client.
"He must first and foremost be allowed to be in contact with other people," Oeystein Storrvik told reporters after the verdict. He declined to say what Breivik`s reaction was to the ruling.
Norway violated mass killer Breivik`s human rights, court rules
21 April 2016
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Norway violated mass killer Anders Behring Breivik`s human rights by keeping him in a "completely locked world" after being sentenced for killing 77 people in twin attacks in 2011, a Norwegian court ruled on Wednesday, AzVision.az reports citing reuters.
