Anders Behring Breivik pleaded not guilty Monday for his massacre of 77 people in Norway last July in a defiant start to his trial that saw him greet the Oslo courtroom with a far-right salute.

While families of the victims fought back tears, Breivik showed little emotion and sat stony-faced as the court heard chilling details of the July 22 killings in Oslo and at a summer camp in the idyllic Utoeya island.

"I acknowledge the acts, but not criminal guilt and I claim legitimate defence," the right-wing extremist, who is accused of "acts of terror", told the court.

Dressed in a suit and wearing a gold-coloured tie, Breivik touched his chest and extended his clenched right fist in front of him as his handcuff were removed on his entry into the courtroom.

In the 1,500-page manifesto he posted online shortly before the July attacks, Breivik described the gesture as "the clenched fist salute" of the Knights Templar organisation, of which he claims to be a member but which the prosecution later argued does not exist.

Lead judge Wenche Elizabeth Arntzen opened the proceedings, which are expected to last 10 weeks and focus primarily on whether Breivik is sane and should be sent to prison or a psychiatric ward.

Breivik told the judges he did not recognise their legitimacy.

"I do not recognise the Norwegian court," he said.

On July 22, Breivik, now 33, killed eight people when he set off a bomb in a van parked at the foot of government buildings in Oslo housing the offices of Labour Prime Minister Jens Stoltenberg, who was not present at the time.

He then travelled to Utoeya island outside Oslo where, dressed as a police officer, he spent more than an hour methodically shooting at hundreds of people attending a ruling Labour Party youth summer camp.

The shooting spree left 69 people dead, most of them teenagers trapped on the small heart-shaped island surrounded by icy waters. It was the deadliest massacre ever committed by a sole gunman.

A number of the victims' bodies were found on the "Love Path" that encircles the island, Prosecutor Inga Bejer Engh told the court.

As prosecutors read out the long list of names of those killed and injured, Breivik listened intently with his eyes lowered, appearing to read from a page before him and sometimes taking notes.

His emotions were at times hard to read, but he could be seen smiling as prosecutors recounted elements of his background.

Four court-appointed psychiatrists sitting in the courtroom -- who have drawn two contradictory conclusions about whether he is sane -- shot glances at Breivik to observe his reactions.

Engh told the court: "The accused has committed very serious crimes of a degree we have not seen in our country in modern times."

He "created fear in the Norwegian population".

In the Utoeya massacre, she said "there was panic and fear of death among children and adults."

"He shot at people who were fleeing or hiding, or who he lured out by saying he was a policeman," noting that most of the 69 dead were killed by bullets to the head.

Most of the shooting victims were teenagers -- 56 of them were under the age of 20 and the youngest victim had just celebrated his 14th birthday, she said.

Several family members of the victims cried quietly as they listened to the long reading of the list of victims.

One family member, a woman wearing a yellow African headdress, wiped away tears and shook her head in disgust and despair.

Breivik has previously described his actions as "cruel but necessary" and claims he acted alone and in self-defence against those he considered to be "state traitors" for opening Norway up to multiculturalism and allowing the "Muslim invasion" of Europe.

He faces either 21 years in prison -- a sentence that could thereafter be extended indefinitely if he is still considered a threat to society -- or closed psychiatric care, possibly for life.

He wants to be found sane and accountable for his actions so that his anti-Islam ideology, as presented in his manifesto, will be taken seriously and not considered the ravings of a lunatic.

Breivik has said that court-ordered psychiatric care would be "worse than death".

During the trial, "he will not only defend (his actions) but will also lament, I think, not going further," Breivik's main defence lawyer Geir Lippestad said last week.

The five judges will have to consider the two contradictory psychiatric evaluations presented to the court, and determine whether he is sane and accountable when they hand down their verdict sometime in July.

Monday's proceedings were broadcast live on Norwegian public television NRK, though the sound was blocked out when the names of the victims and injured were read out.