Anders Behring Breivik told a Norwegian judge on Monday his bombing and shooting rampage that killed 93 people aimed to save Europe from a Muslim takeover, and said that "two more cells" existed in his organization.
Breivik has previously said he had acted alone and police have said they have no other suspects in Friday's attacks.
His remarks were relayed by the judge, Kim Heger, in a news conference held after a closed-door custody hearing.
It was not clear whether Breivik is in fact part of an organization, although he has written about a revival of the Knights Templar, a medieval order of crusading monks.
After the hearing, Heger said he had ordered Breivik detained in solitary confinement for eight weeks, with no letters, newspapers or visits, except from a lawyer.
The detention, in line with a request from prosecutors, will allow them to investigate the case against Breivik.
Jeering crowds awaited Breivik at Oslo District Court.
"Get out, get out!" shouted Alexander Roeine, 24, banging on a car he wrongly believed contained the mass killer. In fact police brought Breivik into the courthouse via a side entrance.
"Everyone here wants him dead," Roeine said, adding that he knew one of the dead and three survivors of the attacks.
Breivik had wanted to explain why he perpetrated modern-day Norway's worst peace-time massacre in public. He was denied a public platform, but the judge, in his news conference, gave an account of what the accused 32-year-old had said.
Heger said Breivik had accused the ruling Labour Party of betraying Norway with "mass imports of Muslims."
He said his bombing of government buildings in Oslo and massacre at a summer camp for Labour's youth wing was aimed at deterring future recruitment to the party.
"The goal of the attack was to give a strong signal to the people," the judge quoted Breivik as saying.
Breivik's custody can be extended before his trial on terrorism charges. Police say the trial could be a year away.
"We want to see him really hurt for what he did," said Zezo Hasab, 32, among a crowd who gave Breivik a furious reception.
After the hearing, a police jeep drove away carrying an unshaven Breivik, with close-cropped blond hair and wearing a red jumper with a lighter red shirt underneath.
He appeared calm and did not try to communicate with journalists standing across the road from an underground garage where he was brought down from the courtroom.
He sat unmoving in the back seat, with a policeman beside him, his head tilted slightly back, before being whisked away.
SILENCE FOR THE DEAD
Norwegians held a minute's silence for Breivik's victims.
"In remembrance of the victims ... I declare one minute's national silence," Prime Minister Jens Stoltenberg said on the steps of Oslo University, flanked by Norway's king and queen.
The silence stretched to five minutes as thousands more stood around a carpet of flowers outside nearby Oslo cathedral. Only squawking seagulls and a barking dog broke the silence.
Cars stopped in the streets and their drivers got out and stood motionless as traffic lights changed from red to green.
"This is a tragic event to see all these young people dying due to one man's craziness. It is important to have this minute of silence so that all the victims and the parents of the families know that people are thinking about them," said mechanic Sven-Erik Fredheim, 36, shortly before the silence.
Breivik planted a bomb on Friday outside Stoltenberg's Oslo office which killed seven, then drove to the wooded island of Utoeya and shot dead 86 at the Labour Party youth camp.
In a rambling 1,500-page tract posted online just before the massacre, Breivik explained how violence was needed to rescue Europe from Islam, immigration and multi-culturalism.
If he survived his assault and was arrested, this would "mark the initiation of the propaganda phase," he wrote.
His lawyer, Geir Lippestad, said: "He has been politically active and found out himself that he did not succeed with usual political tools and so resorted to violence."
The judge's decision to close the hearing to the public followed an outcry from Norwegians enraged at the possibility that Breivik would be allowed a public platform for his views.
A Facebook group called "Boycott Anders Behring Breivik" carried the message: "He has planned this stage, to get propaganda. Do NOT let him get that freedom...Boycott all media describing the Norwegian terrorist and his beliefs."
The maximum jail term in Norway is 21 years, although that can be extended if there is a risk of repeat offences. "In theory he can be in jail for the rest of his life," said Staale Eskeland, professor of criminal law at the University of Oslo.
Norwegian newspapers focused on the victims as shock turns to mourning, giving chilling new accounts of the island massacre and focusing on acts of bravery which saved lives.
The main broadsheet Aftenposten led with "Sorrow unites Norway" and printed a picture of a central Oslo square filled with flowers and lit candles in remembrance of the dead.
Daily Dagsavisen asked "Why didn't you come earlier?" citing screams by youth as police arrived on Utoeya island on Friday -- an hour after they were notified of the shooting.
Police believe Breivik acted alone after losing faith in mainstream parties, even those that have gained popularity and parliamentary seats on anti-immigration policies in otherwise liberal, tolerant European nations, including affluent Norway.
The judge said Breivik's statements required investigation, including his remark about the existence of two more cells.
The attack was likely to tone down the immigration debate ahead of September local elections, analysts said, as parties try to distance themselves from Breivik's beliefs and reinforce Norwegians' self-image as an open, peaceful people. ID:nLDE76N0C6]
Party leaders have agreed to delay the start of campaigning for the polls until mid-August, Norwegian news agency NTB said.
Norway's immigrant numbers nearly tripled between 1995 and 2010 to almost half a million. The sense that many were drawn by Norway's generous welfare handouts helped spur the growth of the Progress Party which became Norway's second biggest in parliament after the 2009 election on a largely anti-immigration platform.
SCHEMING TO "RESIST"
Breivik was once a member of the party, but left complaining it was too politically correct. It was then he began scheming to "resist," burying ammunition more than a year ago, weight-lifting, storing up credit cards and researching bomb-making while playing online war games.
After three months of laboriously pounding and mixing fertilizer, aspirin and other chemicals on a remote farm, Breivik drove a hire car packed with the results to the center of Oslo on Friday, triggering the device outside government offices, killing seven and shattering thousands of windows.
He then drove to Utoeya, 45 km (28 miles) away. Dressed as a policeman, he calmly shot down Labour Party youngsters at a summer camp. His terrified victims tried to hide under beds or in the woods. Some leapt into the lake to escape.
"This is going to be an all-or-nothing scenario," Breivik wrote in his English-language online journal on the morning of the attack. "First coming costume party this autumn, dress up as a police officer. Arrive with insignias:-) Will be awesome as people will be very astonished:-)."
While Breivik was stalking his prey on Utoeya, it took police a full hour to get a team of elite forces to the island after one boat, overloaded with officers and equipment, was forced to stop when it began to take on water.