Anders Behring Breivik cries at own propaganda film Reuters

In an average year, 30 murders are committed in Norway. In three hours one afternoon in July last year, Anders Behring Breivik more than doubled that figure. Yet when he appeared at the first day of his trial on Monday, the 33-year-old insisted he was not guilty of acts of terrorism resulting in the deaths of 77 people.

"I acknowledge the acts," Breivik told Oslo central court when asked to enter a plea. "But I do not plead guilty and I claim that I was doing it out of necessity."

His lawyer had already warned that this would be how Breivik would justify planting an enormous bomb outside the government quarters in Oslo, killing eight people, before heading to the island of Utøya to gun down 69 more attending a summer camp of the ruling Labour party.

Earlier, he announced that he did not recognise the Norwegian court – because, he said, it receives its mandate "from political parties who support multiculturalism".

Breivik was defiant as he arrived in court, giving a closed-fist salute before shaking hands with prosecutors and court officials and then declaring himself a "writer" when asked for his occupation.

Breivik wiped away tears as he watched a trailer for a propaganda film he had made to justify the one-man war which reached its awful conclusion on 22 July.

According to a lip-reader for the Norwegian broadcaster TV2, Breivik is to have said that watching the film made him "emotional".

Breivik has said the attacks were necessary to protect Norway from being taken over by Muslims.

He remained impassive as prosecutors read out the indictment, which detailed not just the crimes for which Breivik was standing trial – charges of terrorism "with the intention of seriously intimidating a population" – but also those he had killed, and how they had died.

It made harrowing listening, the clinical medical details pinpointing how Breivik had ended so many young lives.

The youngest to die on Utøya was Sharidyn Meegan Ngahiwi Svebakk-Bohn, who had just celebrated her 14th birthday. Along with 10 others, she was shot on Kjærlighetsstien, which translates as Lovers' Path.

The prosecutor read: "She was in the area by the escarpment below Lovers' Path and was shot twice with the pistol and/or rifle. One of the shots penetrated inter alia the left lung, the main stem of the pulmonary artery and the aorta. The other shot crushed the 11th chest vertebra and liver, leaving through the right flank. Svebakk-Bohn died of the gunshot injuries to the chest causing internal and external blood loss."

Breivik's crimes were "extremely serious offences on a scale that has never previously been experienced in our country in modern times", said the prosecutor, Svein Holden. The result, he added, has "given rise to serious fear in parts of the Norwegian population".

Much of the morning in court was given over to explaining how, not why, Breivik had planned the attacks.

The five-member panel of judges was told that Breivik's shift towards extremism began in high school when he joined the youth wing of the Norwegian Progress party.

He had told investigators he was a resistance fighter in a far-right militant group modelled after the Knights Templar, a western Christian order that fought during the crusades. He claimed he joined the group after connecting with "militant nationalists over the internet" who eventually invited him to a meeting in London in 2002. But police have found no trace of any organisation and say he acted alone.

"In our opinion, such a network does not exist," Holden said.

The court heard Breivik's initial plan was to be an "economic supporter" of the anti-Islam movement in western Europe. He appeared to stockpile "considerable amounts of money" made from selling fake diplomas and school certificates online.

But in 2006, Breivik folded his company and moved in with his mother in her Oslo flat. It was at this point, say prosecutors, where Breivik decided to become "active" against what he saw as a Muslim takeover of western Europe. The judges were shown a picture of Breivik's bedroom, with the computer where Holden said he played the game World of Warcraft "full time" between the summer of 2006 and 2007.

Working from his mother's house, he composed a rambling manifesto he published online before the attacks. In it, Breivik wrote that "patriotic resistance fighters" should use trials "as a platform to further our cause".

• This article was amended on 17 April 2012. The original said Breivik's justification was self defence, as a result of a mistranslation by court interpreters. This has been corrected.