Anders Behring Breivik has given a Nazi salute upon his return to court to accuse the government of violating his human rights by holding him in isolation.
The Norwegian mass murderer has compared his conditions in prison to “torture”.
Anders Breivik, 37, killed 77 people in 2011 when he bombed central Oslo before going on a shooting spree at a Labour Youth camp on the island of Utoya.
The right-wing extremist was sentenced to 21 years in prison in 2012.
Entering the court in a grey suit shortly before 09:00 Anders Breivik shook hands with his lawyers. He made the salute after police removed his handcuffs.
Anders Breivik accuses the Norwegian government of breaching two clauses of the European Convention on Human Rights.
One of the clauses guarantees the right to respect for “private and family life” and “correspondence” and the other prohibits “inhuman or degrading treatment or punishment”.
His lawyer, Oystein Storrvik, told AFP news agency that Anders Breivik had been “very stressed due to his isolation” in Skien prison, about 60 miles south-west of the capital Oslo.
“One of his main things to do (in prison) was to study and he has stopped that now, and I feel that is a sign that isolation has been negative to his psychological health,” he said.
Authorities say Anders Breivik’s correspondence is censored to stop him setting up an “extremist network”. His visits are almost all with professionals across a glass partition.
The attorney general’s office has insisted that Anders Breivik’s prison conditions are “well within the limits of what is permitted” under the convention.
The court hearing, which is being held in the gymnasium of Skien prison, is expected to run until Friday.
Anders Breivik is expected to testify on March 16.
If the court decides that Anders Breivik’s prison conditions are so strict that they cause him harm and violate his human rights, it could order an easing of restrictions.
In September 2015, Anders Breivik threatened to starve himself to death in protest at his treatment in prison.
His cell at Skien prison has a TV and computer but he has no access to the internet.
In a letter to media outlets in Norway and Sweden, Anders Breivik said he was kept in almost total isolation, with time outside his cell limited to one hour a day.
The mass murderer said the harsh prison conditions had forced him to drop out of a political science course at the University of Oslo.
Anders Breivik was first held at Ila Detention and Security Prison near Oslo before being moved to Skien in 2013.
At Ila, he also complained of being held in “inhumane” conditions.
In a letter to prison authorities, Anders Breivik said his cell was poorly decorated and had no view.
Anders Breivik also complained that his coffee was served cold, he did not have enough butter for his bread, and he was not allowed moisturizer.