"I lifted my weapon and I shot him in the head," Breivik told the court of his first victim on July 22nd on Utøya, where he shot 69 people dead after first killing eight people in a bombing of government buildings in Oslo.
Before launching into the part of his testimony most dreaded by the many survivors and victims' relatives watching his trial, the 33-year-old right-wing extremist warned people who didn't want to hear the "gruesome" details to leave.
He went on to explain that when he arrived on the island, dressed as a police officer, he had been reluctant to go through with his plan, saying that the minute he spent trying to decide whether or not to shoot "seemed like a year."
"My whole body tried to fight against ... there were a hundred voices in my head saying 'Don't do it! Don't do it!'," he told the court.
But after he lifted his gun and shot down his first victims, an off-duty police officer and the camp administrator, there was no more hesitation.
He said he calmly walked up towards a cafe building, which was full of people and where he killed 11.
"I thought: 'Now I am going into that building and will execute as many people as possible in that building'," he told the court coldly, as survivors and family of his victims cried quietly.
Some then got up and left the room.
Breivik, who on Friday was wearing a black suit and shirt with a silver tie, said he didn't remember everything from the shooting spree.
Yet at times he provided devastatingly detailed descriptions, saying in one room he first shot four or five people in a group in the head.
"I think many are screaming and begging for their lives," he said, sneering slightly as he recalled how some of the people in the room were "paralyzed" and did not run away even when he had to reload his gun.
He then turned to another group on the other side of the room.
"I don't know why there were still people in the room at this time," he said, before adding "I shot them all."
Many of the victims received several bullets to the head and back.
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"Some of them were playing dead, that's why I fired so many times."
Survivors and families of victims in the courtroom cried as he recounted each killing. Two women hugged each other as they wiped away tears. Others walked out, unable to take more.
But Tore Sinding Bekkedal, who survived the carnage, listened to every word.
"When you've lived through such a thing, you have a pretty high tolerance for horror," the 23-year-old told AFP.
A lawyer for the families, Frode Elgesem, told reporters: "What perhaps has made this a powerful experience for everybody ... is to see the way he
discusses what he has done."