"The attacks on July 22 were a preventive strike. I acted in self-defense on behalf of my people, my city, my country," Breivik declared, demanding to be found innocent of terror and murder charges.
He set off a bomb outside the government headquarters in Oslo, killing eight, then drove to Utoya island outside the capital and massacred 69 people in a shooting spree at the governing Labor Party's youth summer camp on Utoya island. It was the most "spectacular" attack by a nationalist militant since World War II, he boasted.
The court's main judge interrupted Breivik several times Tuesday, asking him to get to the point, but let him continue after he threatened that he would either finish his speech or not speak at all.
"It is critically important that I can explain the reason and the motive" for the massacre, said Breivik.
Breivik, who has admitted the attacks, said it doesn't matter if he's sent to prison, because living in a country ruled by "multiculturalists" was a prison in itself. The main point of his defense is to avoid an insanity ruling, which would deflate his political arguments.
In his address, he lashed out at Norwegian and European governments for embracing immigration and multiculturalism. He claimed to be speaking as a commander of an anti-Islam militant group he called the Knights Templar - a group that prosecutors say does not exist.
Maintaining he acted out of "goodness, not evil" to prevent a wider civil war, Breivik vowed, "I would have done it again."
Mette Yvonne Larsen, a lawyer representing victim's families, also interrupted Breivik, saying she was getting e-mail and text message complaints from victims who felt the defendant was turning the trial into a platform to profess his extremist views.
But even she showed some sympathy for Breivik's right to explain his actions.
"We understand that the court allows it, but we felt it was our duty as lawyers for the bereaved to intervene," Larsen told reporters.
Norwegian legal experts said it's important that the country's legal traditions apply to everyone, even Breivik, whose massacre shocked the wealthy, peaceful nation. The justice system here isn't about "revenge, but sober, dignified treatment" for everyone accused of a crime, said Thomas Mathiesen, a professor of sociology of law at the University of Oslo.
"It is deeply ingrained in Norwegian tradition and fundamental values. If it lasts all the way through the 10 weeks of this trial, and I think it will, we have an important message to the world," he said.
If found mentally sane - the key issue to be decided in the trial - Breivik could face a maximum 21-year prison sentence or an alternate custody arrangement that would keep him locked up as long as he is considered a menace to society.
If declared insane he would be committed to psychiatric care for as long as he's considered ill.
"What I see happening in Norway with Breivik's statement is a trial about politics, not legal evidence," said Jeff Kass, a reporter who wrote a book about the 1999 U.S. Columbine school shooting.