Norwegian mass killer Anders Breivik says he is a "nice person" who de-humanised his victims to carry out the bombing and shooting rampage that led to the deaths of 77 people.

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The 33-year-old anti-Muslim fanatic told a court in Oslo he did not consider himself criminally insane - the key question at the heart of his trial - but instead viewed himself as basically a "nice person" who had trained himself to stifle his emotions so he could carry out the 2011 attacks.

In testimony that brought tears to the eyes of some in court, Breivik recalled terrified faces as he shot teenagers in a café. He remembered the sight of blood and reloading his weapon although he could not recall all of the details. Breivik said he was in a state of shock as he randomly fired at those who crossed his path, shooting some in the head as they lay paralysed with fear on the ground.

"People were running in all directions," he testified. "I didn't run because I was carrying a lot of equipment."

'Begging for their lives'

In a clinical, emotionless tone, Breivik said he considered torching a building where some victims were hiding but did not have a lighter. Instead, he gunned down people outside who were "begging for their lives".

Earlier Breivik described how easy it was to board the ferry to Utoeya island wearing his fake police uniform and carrying a rifle. He said he hesitated before he started shooting, explaining that he heard 100 voices in his head telling him to stop. Instead, he began a killing spree that would leave 69 dead, most of them teenagers. Another eight people died earlier in the day in a car bombing.

Breivik's testimony upset some Norwegians who tweeted that the news was so difficult to comprehend they turned off reports and took flowers down to the courthouse.

"He has a flak jacket on his emotions," said Asne Seierstad, a Norwegian author and journalist. "Up to now, he has not been moved by anything but his own (anti-immigration) video."

'Nice person'

Breivik said he considered himself "quite normal" until 2006 when he "commenced de-emotionalising" to train for the massacre: "Many people will describe me as a nice person or a sympathetic, caring person to friends and anyone."

Breivik has shown no sign of remorse during the trial, now in its fifth day. He said he "de-humanised" his victims because he feared he would break down if he removed the mental protection he created to separate him from those he killed.

Breivik previously testified that he planned to decapitate former Norwegian Prime Minister Gro Harlem Brundtland but the leader had left Utoeya Island before Breivik massacred people attending a Labour Party youth retreat.

Massacre preparations

In addition to studying al-Qaeda techniques, Breivik said he researched Oklahoma bomber Timothy McVeigh, who set off a fertiliser bomb in 1995, killing 168 people.

Breivik also played online games "Call of Duty: Modern Warfare" and "World of Warcraft", sometimes obsessively. He said he moved home with his mother to save money and spend up to 16 hours a day playing war games.

"It is easy to press a button and detonate a bomb," he said. "It is very, very difficult to carry out something as barbaric as a firearms operation."

If Breivik is found sane, he faces a maximum 21-year prison sentence or a custody arrangement that would keep him locked up as long as he is considered a menace to society. If Breivik is found insane, he could be committed to psychiatric care.

Execute or acquit

Television cameras remain barred from the proceedings during Breivik's testimony to avoid giving him a platform for his views, although reporters are allowed to tweet from proceedings.

A court-appointed team of psychiatrists has already concluded Breivik was psychotic, although a second team found him mentally capable. Breivik asked the court to either execute him or acquit as he would consider a jail sentence "pathetic."

His trial is expected to last 10 weeks.