The right-wing extremist's comments were seen as a strategy aimed at obtaining an easing of the restrictions on his contacts with the outside world, in particular his far-right supporters.
“I've become more radical. I was radical to start but these past five years I've become much more radical,” the 37-year old Norwegian said as he gave evidence against the state, which has appealed a ruling that found it guilty of treating him “inhumanely”, primarily because of his isolation from other inmates.
In July 2011 Breivik, disguised as a police officer, tracked and gunned down 69 people, most of them teenagers, at a Labour Party youth camp on the island of Utøya, shortly after he killed eight people in a bombing outside a government building in Oslo.
He said he killed his victims because they valued multiculturalism.
Found guilty of the bloodiest attack on Norwegian soil since World War II, he was sentenced in 2012 to 21 years in prison, which can be extended indefinitely as long as he is considered a threat.
In prison, Breivik has three cells where he can play video games and watch television. He also has a computer without internet access, gym machines, books and newspapers.
Despite the comfortable material conditions, an Oslo district court last April found the Norwegian state guilty of “inhumane” and “degrading” treatment, in violation of the European Convention on Human Rights.
The court's main argument was his prolonged isolation: he has been held apart from other inmates since 2011 for security reasons.
Breivik's lawyer Øystein Storrvik argued the isolation has rendered his client “mentally vulnerable”.
“I'm seriously affected by the isolation and radicalisation is perhaps the most serious effect of my isolation,” Breivik told the court on Thursday.
His comments were in sharp contrast to the picture painted on Wednesday by the lawyer representing the state, Attorney General Fredrik Sejersted.
He described an inmate in “extraordinary physical and psychological form,” who even wrote a thank you note to prison staff last year.
Psychiatry professor Ulrik Fredrik Malt told Norwegian broadcaster NRK that Breivik's remarks were “purely strategic”.
“If he gets the court to agree that he has been harmed (by the isolation), then they have to give him more contact with others,” Malt said.
The state has argued that Breivik has not been isolated, noting his multiple contacts with guards, with whom he plays backgammon, his lawyers and pastors, and his correspondence with the outside world.
Citing recent psychiatric assessments of Breivik, Sejersted insisted the killer was still highly dangerous, which justified the strict prison regime and limited contacts with the outside world, aimed at preventing him from building networks capable of carrying out new attacks.
On Thursday, Breivik told the court he had decided not to use his lawsuit as a platform to spread his white supremacist ideology, as he has done in the past to the dismay of survivors and victims' families.
He nonetheless made a raised-arm Nazi salute on Tuesday at the opening of the appeals hearing, and presented himself to the court as the secretary of a non-existent political party.
Despite repeated questions from the Attorney General, he also refused to express remorse for the 2011 attacks.
As Norway tries to turn the page on the deeply traumatic episode, a support group for victims' families called on media to exercise restraint in their coverage of the hearing.
“It's particularly difficult for a lot of us when the man behind these crimes gets so much attention in the media,” the group said on its website.
The hearing has been broadcast live in Norway, but the judge on Thursday barred the media from airing Breivik's remarks.
The court is also to rule on an appeal by Breivik regarding his inability to freely communicate with the outside world. On that point, the judge in April ruled in favour of the state. Breivik insists that violates his right to privacy under the European Convention.
The six-day hearing is being held for security reasons in the Skien prison in southern Norway where Breivik is incarcerated.
It is scheduled to wind up on January 18, and a ruling is due in February.