Breivik, in the fifth day of his trial in Oslo, spent 90 minutes talking about the massacre at a Labor Party summer camp on Utoya island last July 22. Thirty-three of the victims were less than 18, the youngest, 14.
"You are going to die today, Marxists," Breivik said he shouted as he fired a semiautomatic rifle and handgun, reports in British newspapers The Daily Telegraph and The Guardian said.
Breivik also has admitted setting off a bomb in an Oslo government building that killed eight others and said he would not have gone to the island if the bomb had done more damage.
He said he pretended to be a police officer to gain admission to the island.
"There was complete chaos and people running in all directions. I thought, 'I'm going to enter that building and execute as many people as possible,'" Breivik testified. "I started to just fire and I shoot people in the head. I guess I shoot four or five."
Breivik said he was surprised some of his victims were paralyzed by fear -- even when he ran out of ammunition and was reloading -- and many begged for their lives. He said he could tell when people were playing dead and shot them to finish them off.
The Guardian reported many of those in the courtroom had tears in their eyes as Breivik talked. Before he began describing the events, he warned spectators they might want to leave the courtroom because his testimony would be "horrendous."
Outside, people brought flowers for a makeshift shrine.
Testimony was to continue Monday.
Earlier, Breivik said many people "will describe me as a nice person, a sympathetic person, who is very caring to close people and friends."
Breivik has justified the killings as self-defense against multiculturalism.
He said "cultural Marxists" had subverted traditional gender roles so "suddenly boys are supposed to start knitting and doing crochet."
Breivik said he is "of sound mind" and it's important to see the difference between political extremism and madness.
Violence, he said, was a last resort after he tried getting his message out through "normal involvement through political parties, writing essays, communicating or writing comments on the Internet."
Just before he began shooting, Breivik said, "I so didn't want to do this."
With a gun in his hand, he said, "It was like 100 voices in my head saying, 'Don't do it, don't do it.'"
Before shooting, he said he thought, "It's now or never."
Breivik faces a maximum prison sentence of 21 years if found sane or if held in alternative custody could be locked up as long as he's deemed a threat to society, the Guardian said.
If he is found to be insane, he would be committed to psychiatric care for as long as he's considered ill.