Anders Behring Breivik called the police to surrender, but then decided to carry on killing "till I die" when they failed to return his call, he has told his trial in Oslo.
He said his plan was to kill as many people as possible.
But he said he had tried to spare some of the young people caught up in the killings on Utoeya island who he thought were under 16.
Breivik has admitted the Oslo bombing and island shootings that killed 77.
The trial will decide whether he is sane. A state psychiatric commission requested further clarification on the second of two psychiatric reports, which concluded he was sane and accountable for his actions.
The first report found him legally insane.
Breivik said he would do "anything to prevent" committal to a psychiatric hospital.
Earlier, he said he had "lost absolutely everything" on 22 July 2011, all his family and friends, therefore he understood the loss he imposed on others.
But he said the killings were "a small barbarian act to prevent a larger barbarian act".
In July last year, Breivik set off a car bomb near government buildings in Oslo, killing eight, and then massacred 69 participants in a Labour Party youth camp on the nearby island of Utoeya.
As he continued to describe the events on Utoeya island, Breivik told the court he had made an impromptu call to police with the intention of surrendering.
But the police did not call him back, he said.
He added that he was thinking: "I will carry on until I die."
Breivik apologised to those he did not define as his enemies.
He also said he did not kill "many" under the age of 16 and chose not to shoot three people because they appeared to be minors.
But he maintained he was not sorry for killing 77 and injuring hundreds of others in the twin attacks last year.
And he described walking through the bodies lying on the ground, meticulously shooting each one again to ensure they were dead.
For the first time, Breivik apologised not to those he killed, but to some of those he injured in the bomb attack on Oslo government buildings, whom he considered not to represent his self-defined enemy.
Later, he said he would have carried out the attacks again "even knowing that 40% of the people on Utoeya were under 18 years old".
He added there were many others in Norway "who deserve to be executed, eg journalists and members of parliament".
During his five days of giving evidence, Anders Behring Breivik has usually answered questions from both prosecution and defence in a polite manner, reports Lars Bevanger in Oslo.
On Monday, however, he appeared defiant on several occasions, especially when the prosecution team asked him about details of his so-called manifesto.
He repeatedly interrupted and said they were out to "ridicule" him.
Breivik said he understood his actions would lead to sympathy for the Labour party, but believed that sympathy could be eroded in two to three years.
He again drew comparisons to al-Qaeda, arguing the Madrid train attacks in 2004 led to a change of government and the pull-out from Afghanistan.
"Militant nationalists have a lot to learn from [al-Qaeda]," he told the court.
In his other evidence on Monday, Breivik said he:
believed political leaders would be "emotionally unstable" and would instruct police to execute him after his arrest at Utoeya
told police he believed his family might be executed after his actions
considered at one stage stealing a small plane from a nearby airfield to flee after his actions
planned to make a video recording of himself decapitating former PM Gro Harlem Brundtland while reading a prepared text spelling out her "crimes"
repeatedly replied "no comment" to any questions about other members of an anti-Muslim network called the Knights Templar, which Breivik says he belongs to but which prosecutors believe does not exist
Breivik denies criminal responsibility for killing the 77 victims, saying he was defending Norway from multiculturalism.
He said he had envisaged the most important attack as being the Oslo bombing, but Utoeya "became the most important attack when the government building did not collapse" as planned.
Depending on whether he is found sane or not, he faces either prison or committal to a psychiatric institution.
Breivik himself maintains he is sane, but a practitioner of political extremism.
Breivik has been allocated five days in total to give evidence, with the entire proceedings expected to last 10 weeks.
Monday was expected to be his last day of evidence, but he may return to the stand on Wednesday when his psychological state is discussed.
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