It was the first time in nearly four years that Breivik, 37, has appeared in public since he was convicted in a 2011 bombing and shooting spree that has been called the deadliest massacre in Norway since World War II.
On July 22, 2011, Breivik killed eight people when he bombed Oslo's government district and then slaughtered 69 others — mostly teenagers — gunning them down at a summer camp for the Labor Party’s up-and-coming leaders, according to Reuters.
Breivik was convicted of terrorism in 2012 and given the country's maximum sentence: 21 years behind bars, although the sentence can be extended.
"The plaintiff has not shown any sign of remorse," government attorney Marius Emberland said during opening remarks, according to the Associated Press. "Breivik is a very dangerous man."
Breivik, who has been held in isolation in Norway's Skien prison, not far from Oslo, filed a lawsuit, claiming his time in solitary confinement is inhuman.
Authorities said Breivik has been kept in a three-room prison cell — with an area for sleeping, an area for studying and an area for working out — where he can run on a treadmill, watch DVDs or play games on a Sony PlayStation, according to the New York Times.
He has also been taking distance-learning courses at the university.
Breivik on Tuesday entered a makeshift courtroom inside the prison's gymnasium — where the case is being heard for security reasons — to complain about his rights.
His attorneys are arguing that the Norwegian government has violated two clauses in the European Convention on Human Rights — one that upholds a person's right "to respect for his private and family life, his home and his correspondence" and another that prohibits "inhuman or degrading treatment or punishment," according to reports.
"This case is about something much more than what many people think, just a lawsuit brought to allow Breivik back into the spotlight to explain himself," his attorney, Oeystein Storrvik, said, according to the Guardian.
"This case is simply about his detention conditions for the rest of his life."
Breivik's attorney called into question excessive strip-searches, frequent handcuff usage and, namely, isolation — all of which Breivik has said causes him "mental strain." The goal in the trial, his attorney said, is to give Breivik contact with other prisoners and lessen restrictions on his communication with the outside world, according to the Associated Press.
Government attorneys said 600 letters have been withheld from about 4,000 letters either sent by Breivik or to him, according to the news agency, to keep him from establishing an "extremist network."
"He is a citizen of Norway and even though he is convicted for a horrible crime, he hasn’t lost his human rights," said Ina Stromstad, a spokeswoman for the Olso district court.
Breivik will testify Wednesday in his case.