Anders Behring Breivik has wept in court as an anti-Islam propaganda video he created was played - but showed no emotion when claiming he killed in self-defence.
At the start of his trial in Oslo, Breivik made a far-right salute and said he did not recognise the legitimacy of the court.
He pleaded not guilty to charges of terrorism and murder, but said he "acknowledges the acts" and that he killed 77 people last July in "self-defence".
The 33-year-old smirked as the prosecution spoke about his online gaming habits and bomb-making plans - but was tearful as the court was shown the 12-minute video he posted online before the terror attacks.
A lip-reading expert for Norwegian broadcaster TV2 said Breivik told his lawyer: "I am okay. It is just an emotional film."
Defence lawyer Geir Lippestad later said, while the court must considered Breivik's mental state at the time of the attacks, it was the defendant's wish to be sentenced as legally sane.
As a result, Mr Lippestad said his case would support this choice.
Breivik has admitted detonating a bomb in the centre of Oslo and opening fire on young people attending a summer camp on Utoya island nearly nine months ago.
He believes the attacks were a political act designed to prevent what he describes as an Islamic invasion of Norway.
In court, Breivik described himself as a "writer" and told the judges: "I do not recognise the Norwegian courts. You have received your mandate from political parties which support multiculturalism."
Last week, his lawyer told reporters Breivik believes he should be facing a military tribunal , not a criminal court.
The trial began with the prosecution reading out the names and details of those killed and injured in the bombing and shooting spree.
Breivik was then asked what his plea would be. He told the court: "I admit to the acts, but not criminal guilt." He will take the stand again on Tuesday.
The prosecution began its case by giving details about the defendant's life in the years before the crime.
Svein Holden told the court how Breivik made money by making fake diplomas and certificates via his company Diplom Service before 2006, and described his extensive online gaming habits - mostly playing World of Warcraft.
He then gave details about the accused's involvement with the Knights Templar - a network, the prosecutor said, that as Breivik described it, did not exist.
Breivik made Knights Templar uniforms for himself and bought guns and other items connected to weapons, the court heard.
Mr Holden then outlined Breivik's plans for a bomb, with the court shown an image drawn by the accused for the police of the explosive device he built.
He rented a farm 90 miles from Oslo, where he wrote his manifesto and started to assemble the bomb from fertilisers, diesel and chemicals, the court heard.
Discussing Breivik's political ideology, Mr Holden introduced the propaganda video - a summary of his manifesto beliefs - which Breivik uploaded to YouTube.
Mr Holden said the prosecution had "some doubts" as to how the video should be presented to the court - but said the montage of images and thoughts on the "Islamisation of Europe" would be useful in assessing Breivik's court statement, which is due on Tuesday.
After a break for lunch, the court heard details about Breivik's bombing of Oslo's government district, which killed eight people and injured more than 200.
At first, an animation showing the defendant's movements before and after he planted his home-made bomb was played, before actual footage of the explosion, which was deemed not suitable to be broadcast by the media, was shown.
Breivik was again expressionless as he watched the surveillance footage of the bombing.
Both sides agree that on July 22, 2011, he drove the device into the government quarter of the capital just after 3.15pm, parked it in front of the building which housed the prime minister's office, lit a seven-minute fuse, then walked to a getaway car.
Breivik then made his way to Utoya island, where the youth wing of the country's Labour Party was holding its annual summer camp.
After convincing organisers on the mainland that he was a policeman sent to secure the island after the bomb blast in Oslo, he made the short trip over the water.
When on Utoya, he began to shoot dead whoever he saw and for over an hour he aimed, fired and reloaded as terrified youngsters ran for cover.
The court heard an emergency phone call made by a young girl hiding on the island, telling police what was happening as shots rang out in the background - again, it was too distressing to be broadcast.
Breivik surrendered after officers finally arrived on the island, having called police to say that his "mission" had been "accomplished".
The 10-week trial, which is being held in a specially-built courtroom that includes sheets of toughened glass behind the defendant, will hear from eyewitnesses, survivors and forensic experts.
Defence lawyer Vibeke Hein Baera has told Sky News part of their strategy is to call a series of extremists as witnesses, to question an initial psychiatric report that deemed Breivik to be criminally insane.
A second report disputed those findings and a panel of two professional and three lay judges will have to decide whether Breivik should be sent for treatment at a psychiatric hospital or jailed.
If Breivik is found sane, he faces just 21 years in prison for the killings - though the sentence may be extended indefinitely if he is still considered a threat to society.
If he is found insane, he could spend the rest of his life on the closed psychiatric ward, a fate he has declared would be "worse than death".
He wants to be found sane and accountable for his actions, so that his anti-Islam ideology - presented in the 1,500-page manifesto he published online just before the attacks - will be taken seriously and not considered the ravings of a lunatic.
The massacre shocked normally tranquil Norway , home of the Nobel Peace Prize, sparking emotional displays of national unity and a deep reflection on the delicate balance between openness and security.