Breivik rejects criminal guilt for the rampage on July 22, saying the victims had betrayed their country by embracing immigration.
Even the defense admits there is virtually no chance of an acquittal, so the key issue to be determined in the trial is whether Breivik is criminally insane.
Two psychiatric examinations reached opposite conclusions on that point. In a statement to the court, the Norwegian Board of Forensic Medicine asked for additional information from two pscyhiatrists who found Breivik sane, saying their report was incomplete.
Breivik himself insists he is sane, and accuses the prosecutors of trying to make him look irrational.
"I know I'm at risk of ending up at an insane asylum, and I'm going to do what I can to avoid that," he told the court.
Breivik became defensive as prosecutors quizzed him about sections of the 1,500-page manifesto he posted online before the attacks. It describes uniforms, medals, greetings and codes of conduct for the "Knights Templar" militant group that he claims to belong to. Prosecutors don't believe it exists.
In one section, read by prosecutor Svein Holden, Breivik speculated that in his future society, the loyalty of potential knights might be tested by asking them to undergo surgical amputation and castration. Breivik chastised the prosecutor for what he called "low blows" and said the segment was taken out of context.
Breivik, 33, showed no remorse as he continued his shocking testimony about his shooting spree at the annual summer youth camp of the governing Labor Party.
Calling the rampage "necessary," Breivik compared being shunned by those close to him to the grief of the bereaved.
"The only difference was that for my part it was a choice," he said.
The self-styled crusader apologized to the family of a pub owner who was among the eight people killed in the blast outside the government offices in Oslo, saying it was not his intention to kill "civilians."
Holden asked him if he wanted to express a similar apology to the families of the other victims, including the 69 killed on the youth camp on Utoya island.
"No I don't," Breivik said. "Utoya is a political indoctrination camp."
"I see all multicultural political activists as monsters, as evil monsters who wish to eradicate our people, our ethnic group, our culture and our country," he said.
Jon Hestnes, who heads a support group for victims' families and survivors, told The Associated Press it was "gruesome" to listen to Breivik's apology.
"It's an insult to the 76 other people who actually died because of that man," Hestnes said.
"He's not in our world. He isn't, and he doesn't have humanity at all. The way I slap little mosquitoes in the summer, that's how he is about human lives," Hestnes said.
Speaking calmly, Breivik said he used a handgun to kill victims if the distance was less than 10 meters. Otherwise he used his rifle.
Asked why he spared one man who survived the shooting spree, Breivik said he thought it was because the man's appearance made him look "right wing-oriented."
"When I looked at him I saw myself," Breivik said. "I think that was the reason that I didn't fire shots at him."
If found sane Breivik would face 21 years in prison, though he can be held longer if deemed a danger to society. If sentenced to psychiatric care, in theory he would be released once he's no longer deemed psychotic and dangerous.