Anders Behring Breivik has pleaded not guilty to charges of ''acts of terror'' over the massacre of 77 people in twin attacks in Norway last July, saying he was acting in self defence.

"I acknowledge the acts, but not criminal guilt and I claim self-defence," he told the court on the first day of his 10-week trial.

The judge then entered the plea as "not guilty."

He told Oslo District Court that he does not recognise its legitimacy.

"You have received your mandate from political parties which support multiculturalism", Mr Breivik told the court. "I do not acknowledge the authority of the court." he said.

Mr Breivik also said he did not recognise the authority of Judge Wenche Elisabeth Arntzen, because he said she is friends with the sister of former Norwegian Prime Minister and Labor Party leader Gro Harlem Brundtland.

Mr Breivik made a far-right salute as he entered the courtroom this morning.

In the 1,500 page manifesto Mr Breivik posted online shortly before the 22 July attacks, he described the far-right salute as "the clenched fist salute" of the Knights Templar organisation.

Police ushered the defendant, who was wearing a black suit and had his blond hair shortly cropped, to his seat at the front of the courtroom, under the gaze of survivors and family members of those he killed and hundreds of journalists from around the world.

Mr Breivik has presented himself as a ''writer'' at the trial.

He was in tears as the court viewed a propaganda film he made.

Mr Breivik is accused of setting off a bomb in downtown Oslo, killing eight, before shooting dead 69 people, mostly teenagers, at a Labor Party youth camp on Utoya island.

Since Mr Breivik has confessed to the attacks - claiming they were necessary to protect Norway from being taken over by Muslims - the key issue that remains unresolved is his mental health.

The 33-year-old Norwegian was found insane in one examination that recommended committing him to compulsory psychiatric care, while a second more recent assessment found him mentally competent to be sent to prison.

It is up to the judges in Oslo's district court to decide which diagnosis they find most believable.

If deemed mentally competent, he would face a maximum prison sentence of 21 years or an alternate custody arrangement under which the sentence is prolonged for as long as an inmate is deemed a danger to society.

Police have sealed off streets around the court building, where journalists, survivors and relatives of victims can watch the proceedings in a 200-seat courtroom built specifically for the trial.

Thick glass partitions have been put up to separate victims and their families from the defendant.

Norway's NRK television will broadcast parts of the trial, but is not allowed to show Mr Breivik's testimony.

In a manifesto he published online before the attacks, Mr Breivik wrote that "patriotic resistance fighters" should use trials "as a platform to further our cause."

Mr Breivik surrendered to police one hour and 20 minutes after he arrived on Utoya.

The police response was slowed by a series of mishaps, including the lack of an operating police helicopter and the breakdown of an overloaded boat carrying a commando team to Utoya.

Mr Breivik claims he targeted the government headquarters in Oslo and the Labor Party youth camp to strike against the left-leaning political forces he blames for allowing immigration in Norway.

Mr Breivik told investigators he is a resistance fighter in a far-right militant group modelled after the Knights Templar medieval crusaders, but police have found no trace of the organisation and say he acted alone.

Anxious to prove he is not insane, Mr Breivik has called right-wing extremists and radical Islamist to testify during the trial, to show that there are others who share his view of clashing civilisations.

His defence lawyer, Geir Lippestad, said Mr Breivik's only regret is that the death toll was not higher.

"It is difficult to understand, but I am telling you this to prepare people for his testimony," Mr Lippestad said.