The self-confessed author of the attacks that killed at least 93 people and wounded nearly 100 more in Norway is set to appear before a judge for the first time, but could still be weeks away from facing formal charges.

Anders Behring Breivik, 32, is due to appear in court in Oslo on Monday (local time) after telling police that Friday's bombing and shooting attack was "cruel" but "necessary".

Breivik's lawyer told media his client had "admitted responsibility" for the twin attacks, but felt that "what he has done does not deserve punishment".

Breivik will be brought to a central Oslo court house for the first time since his arrest on Friday early evening. His lawyer, Geir Lippestad, says Breivik wants to be allowed to wear a uniform when he faces court, but does not know what uniform his client has in mind.

The judge is first expected to rule on whether the hearing will be open to the public.

Ahead of Breivik's court appearance a minute's silence was held across the country for the victims of the attacks.

The judge will then decide on Breivik's custody conditions while awaiting formal charges. Those charges will only be read out to the accused at a later hearing closer to actual trial.

Under Norwegian law, authorities can keep a suspect under such provisional detention for a period of up to four weeks, which can be renewed.

A rambling 1,500-page tract apparently written by Breivik has surfaced on the internet, saying he had been preparing the "martyrdom operation" since at least late 2009.

Breivik's internet manifesto is part diary, part bomb-making manual, and part a political rant in which he details his anti-Islamic beliefs, his hatred of Marxism, and his initiation into the Knights Templar.

One entry reads: "Friday, July 22. Day 82, initiate blasting sequences at predetermined sites. Have enough material for 20 blasts. I believe this will be my last entry."

The manifesto explains how he set up mining and farming businesses as a front to prepare for the attacks.

"The reasoning for this decision is to create a credible cover in case I am arrested in regards to the purchase and smuggling of explosives or components to explosives - fertiliser," says the tract.

The bombing and shooting spree targeted government offices in downtown Oslo and youth members of the country's governing Labour party.

At least seven people were killed in a car bomb blast outside government buildings in Oslo before Breivik shot 85 people dead at a Labour Party youth camp on the nearby island of Utoya.

About 650 people were on the island when Breivik, wearing a police uniform, opened fire.

Police said it took them an hour from when they were first alerted to reach the island and stop the massacre, the worst by a single gunman in modern times.

Soldiers continued to stand guard in the middle of the capital, Oslo, on Sunday and police said there were still explosives inside Oslo city buildings. Some bodies have not been removed because the buildings are too unsafe.

Norwegian massacre suspect Anders Behring Breivik pictured in a YouTube video. ( AFP )

Nation in mourning

As harrowing testimony emerged from the summer camp where scores of youngsters were mown down, Norway was struggling to understand how a country famed as a beacon of peace could experience such bloodshed on its soil.

Prime minister Jens Stoltenberg and Norway's King Harald V and Queen Sonja led the nation in mourning at an emotional memorial mass in Oslo Cathedral for the victims.

"Many of those who have died were friends," Mr Stoltenberg said.

"I know their parents and it happened at a place where I spent a long time as a young person... It was a paradise of my youth that has now been turned into hell."

The leader of the Labour Party's youth group, Eskil Pedersen, wept openly during the service.

Mr Stoltenberg told the hundreds of mourners that the "scale of the evil" would only fully emerge when the names and photographs of the mostly teenaged victims were published.

Police have not ruled out the involvement of a second gunman, amid witness reports of a possible second shooter on the island.

The death toll could rise further as the search continued for four or five people still missing from the island, aided by a mini-submarine and Red Cross scuba divers.

The farm house of Anders Behring Breivik at Aamot in Hedmark, eastern Norway. Behring Breivik is the primary suspect held in connection with the bomb blast in downtown Oslo and the killing spree on the nearby Utoya island on July 22, 2011. ( AFP: Jo E. Brenden )

'Biggest monster ever witnessed'

Also in the manifesto Breivik foreshadows his infamy, writing: "I will be labelled as the biggest monster ever witnessed since World War II."

Another entry titled "Autumn 2009 - Phase Shift" explains how he set up front mining and farming businesses to prepare the attacks.

Police on Sunday detained several people in a swoop on an organic farm about 160 kilometres north of Oslo.

The farm is suspected of being a cover for Breivik while he was planning the attacks and possibly building the bomb that ripped through central Oslo.

"No explosives were found at the location and those detained have been released," Oslo police said in a statement following the raid in the Sletteloekka district of the capital.

"Police have nothing that could enable these people to be connected with acts of terror."

Policemen, one wearing blue overalls and a surgical mask hanging below his chin, and a soldier dressed in army fatigues paced across the yard and in and out of a red barn facing the small white farmhouse.

Local police chief Ove Osgjelten allowed a small group of reporters to enter the perimeter marked by police tape to view and photograph the farmyard from a short distance.

He declined to give details of the police operation under way.

In front of one outbuilding, along a road leading down a gentle slope to the house, stood a half a dozen thick white sacks of fertiliser. Many fertilisers contain a substance that can also be a key ingredient in a forceful explosive.

ABC/wires