The Norwegian who massacred 77 people to protest against Muslim immigration to Europe has said he had hoped to kill as many as 150 and kept on killing because police failed to respond urgently to his phone call.
Breivik has given a detailed account of his car bomb attack at government headquarters in Oslo on July 22, which killed eight people, followed hours later by his shooting of 69 people, mostly teenagers, at a Labour Party island camp.
He said overnight (NZ time) his "gruesome" actions were to prevent a civil war caused, he said, by a Muslim takeover of Europe.
"This was a minor barbarity to prevent a larger one", he said on the sixth day of a trial that has transfixed Norway.
"I've never ever experienced such a horrendous ... gruesome act as this. But it was necessary," Breivik said in his usual tone, lacking emotion. "It was much more cruel than I expected."
Breivik said he thought that at least another 150 people had drowned in a lake as they fled his gunfire so he called police to surrender, only to find himself forced to leave a message.
"I said 'call me back when you got the right person'," Breivik said. "I told myself 'I will continue until the phone rings'. I thought, I will continue until I die. What would I have done, sat by the pier waiting?"
Breivik has denied criminal guilt, insisting his victims were "traitors" whose multiculturalist views facilitated what he saw as a de facto Muslim invasion of Europe.
Most Norwegians have reacted with horror to his testimony, delivered in a cold, matter-of-fact manner, while there is wide public acceptance of his right as a defendant to give it.
Breivik has had almost free rein to issue warnings against immigration and explain how he scoured the Internet for bomb-making information while writing a 1500-page document declaring himself part of a secretive group that is Europe's answer to al Qaeda - a group the police have said likely does not exist.
Breivik said he spared some people, including a 10-year-old boy whose father was his first victim, and a Labour Party activist because he looked right-wing.
"Some people have the type of look that is associated with the leftist movement," Breivik said.
"This person, (Adrian) Pracon appeared right-wing, that was his appearance. That's the reason I didn't fire any shots at him," said Breivik, 33, whose sanity or lack of it is a prime issue to be determined in the trial.
The 22-year-old Labour party youth wing activist earlier told Reuters: "I remember him pointing the gun at me for quite a long time before he took it down, turned and walked away."
Breivik told how he used a fake police uniform to trick people into coming out of hiding and then shot them at close range.
"I started with ‘have you seen him, do you know where the shots came from?' ... then I said ‘there's a rescue boat that's going to take you to safety but you need to come out'," he said.
Later in the rampage, which lasted more than an hour, Breivik came upon Pracon again as he played dead, and this time shot the son of Polish immigrants through the shoulder.
He said he spared the boy's life because "I could not understand what such a little boy was doing at a political indoctrination camp."
But he said he had no qualms about killing teenagers. His victims were as young as 14. "They were not children, under the legal definition only under 14 are children ... they were political activists," he said.
Breivik issued his first seeming apology, to innocent bystanders hurt or killed when his 950kg fertiliser bomb went off in Oslo. More than 200 were injured.
"To all of those ... I want to say I am deeply sorry for what happened," he said. "But what happened, happened."
Ahead of the trial, which is expected to last 10 weeks, one court-appointed team of psychiatrists concluded that Breivik was psychotic while a second found him mentally capable.
If Breivik is deemed sane, as he hopes to be, he could face a 21-year prison sentence with indefinite extensions for as long as he is considered dangerous.