Anders Behring Breivik has told his trial that he believed he could tell the ideology of his victims by looking at them, and tried to spare one who appeared "right-wing".

On the sixth day of his trial, Mr Breivik said "certain people looked more leftist than others."

He explained how he picked off "Marxists" with his rifle and pistol while passing over a young man he thought looked conservative.

"This person ... appeared right-wing; that was his appearance. That's the reason I didn't fire any shots at him," said Mr Breivik, 33, whose sanity is a prime issue to be determined in the trial.

The 22-year-old he chose not to kill, a Labour Party youth wing activist named Adrian Pracon, has told Reuters:

"I remember him pointing the gun at me for quite a long time before he took it down, turned and walked away."

Mr Breivik has given a detailed account of his car bomb attack at government headquarters in Oslo, which killed eight people, and a follow-up gun massacre at a Labour Party island camp where he killed 69 people within a few hours on 22 July.

Most Norwegians have reacted with contained horror to the content of Mr Breivik's testimony, while there is wide public acceptance of his right as a defendant to give it.

He has had almost free rein to issue warnings against immigration and explain how he scoured the internet for bomb-making recipes while writing a 1,500-page document declaring himself part of a secretive group that is Europe's answer to al-Qaeda - police do not believe the group exists.

Mr Breivik has denied criminal guilt, insisting that his victims were "traitors" whose multi-culturalist views facilitated what he saw as a de facto Muslim invasion of Europe.

This morning he issued his first apology to innocent bystanders hurt or killed when his 950kg fertiliser bomb went off in Oslo. 

"To all of those ... I want to say I am deeply sorry for what happened," he said. "But what happened, happened."

He called his acts "a minor barbarity to prevent a larger one", apparently referring to Europe's supposed cultural decline.

Ahead of the trial, which is expected to last 10 weeks, one court-appointed team of psychiatrists concluded Mr Breivik was psychotic while a second found him mentally capable.

If Anders Behring 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.