The last day of the trial of Anders Behring Breivik, who killed 77 people in Norway last July, has ended with his defence, as expected, calling for his acquittal.

Even though there is no chance Breivik will be set free, Geir Lippestad, his main lawyer, formally made the request since Breivik had pleaded not guilty, despite having confessed to carrying out deadly twin attacks on July 22 last year.

Breivik evoked the "principle of necessity", claiming his attacks were "cruel but necessary" to protect Norway against a "Muslim invasion". 

As Breivik prepared to begin his last statement, a number of people walked out of the courtroom.

"We have no need to hear more about what he has to say," Trond Henry Blattman, leader of a victim's support group, told reporters.

"We have heard him many times, we don't hear anything new ... we want to show that we don't care about what he has to say, who he is, what he has done."

'My ethnic group'

Breivik used the 45 minutes accorded him to make final remarks on Friday to claim that his attacks were necessary in defence of "my ethnic group" against multiculturalism, and demanded his acquittal.

Shortly before the trial day began, another defence lawyer, Vibeke Hein Baera, told the AFP news agency of the request for acquittal that Breivik "knows that this is just a formality that is far from any plausibility". 

Lippestad used most of his closing argument to attempt to prove that the 33-year-old right-wing extremist is criminally sane, and should be sent to prison, not a closed psychiatric ward as requested by the prosecution.

On the day of the killings, Breivik first set off a car bomb outside government buildings in Oslo, killing eight people, before travelling to Utoeya island, northwest of the capital.

There, he spent more than an hour methodically shooting and killing another 69 people, mostly teenagers.

The victims, the youngest of whom had just celebrated her 14th birthday, had been attending a summer camp hosted by the governing Labour Party's youth organisation.