Anders Berhing Breivik
Anders Berhing Breivik Retuers

Attendees at the trial of Norwegian mass murderer Anders Behring Breivik were warned that Friday would likely the "the hardest day" yet.

On Thursday, the 33-year-old self-styled anti-Muslim militant had many in the Oslo district court in tears as he detailed how he set off a car bomb at the foot of a government building on July 22. The bomb killed eight people.

Breivik also described how he had wanted to kill everybody in the building, including the entire Norwegian government.

He had initially wanted to use three bombs – including one targeting Norway's royal palace – but said he altered his plan due to the difficulty in constructing the explosive devices.

However, defense attorney Geir Lippestad warned, after the end of court on Thursday, that “tomorrow will probably be the hardest day.”

Breivik told the court Thursday that he had also intended to kill all 569 people on Utoeya Island when he carried out his shooting massacre the same day.

On Friday, he is set to describe in detail how he killed each of the 69 people — most of them teens — that day on the island.

As the trial day began, main judge Wenche Elizabeth Arntzen reminded survivors and family members of the victims that they were allowed to leave the courtroom if they felt they needed to do so.

Breivik, charged with “acts of terror”, has confessed to killing 77, but entered a plea of not guilty saying his actions were “cruel but necessary."

The key point of contention between Breivik and prosecutors is whether he is criminally insane.

Prosecutors maintain the militant Knights Templar group Breivik describes in his 1,500 page manifesto does not exist and is a product of his delusional perceptions of reality.

If found sane, Breivik could face a maximum 21-year prison sentence – which he described as "pathetic" – or an alternate custody arrangement that would keep him locked up as long as he is considered a menace to society.

Breivik, who became visibly agitated when challenged by prosecutors about the Knights Templar on Wednesday, insists the group exists.

If declared insane, he would be committed to psychiatric care for as long as he's considered ill. It would also allow authorities to dismiss his radical right-wing ideology as the ravings of a madman.