Anders Behring Breivik has returned to court in Oslo for what is scheduled to be his final day of evidence on last July's massacre in Norway.
He has already admitted to the Oslo bombing and island shootings that left 77 people dead. The main aim of the trial is to decide whether he was sane.
On Friday, he described shooting people who were "begging for their lives".
The 33-year-old said he was normally a nice person, but had shut off his emotions to carry out the attacks.
On 22 July, 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.
'Moment of hesitation'
In Friday's testimony, he said he had arrived on the island dressed as a policeman, and told security officials at the camp he had been posted there following the bombings in the capital.
Before shooting his first victims, Breivik said he had "100 voices" in his head telling him not to do it.
But after that moment of hesitation, he said he pulled the trigger, shot two people in the head and moved on.
He said he reloaded after running out of ammunition. "People were begging for their lives. I just shot them in the head."
Others pretended to be dead, he added, but he knew they had not been wounded and shot them too.
Breivik continued his rampage around the island, luring youths from hiding places by telling them he was a police officer who was there to protect them.
The BBC's Steven Rosenberg, who was in court on Friday, said the stunned silence in the courtroom turned to tears as Breivik's testimony unfolded.
Breivik admits killing all 77 victims but denies criminal responsibility, 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.
In earlier statements to the court, he insisted he was "under normal circumstances a very nice person, very caring about those around me".
He said he "absolutely" understood why his testimony was horrifying to others.
But said he had embarked on a deliberate programme of "dehumanisation" in 2006 to prepare to carry out killings.
He added that empathy was not possible, as he would "break down mentally" if he tried to comprehend what he had done.
Asked if he could feel sadness, he said "yes", saying the funeral of a friend's brother had been his "saddest day".
Breivik has been allocated five days in total to give evidence, with the entire proceedings are expected to last 10 weeks.