After a closed-door hearing Monday, Judge Kim Heger said Anders Behring Breivik acknowledged carrying out the attacks -- a bomb blast in downtown Oslo and a shooting spree nearby on Utoya Island -- but said the attacks were necessary to prevent the "colonization" of Norway by Muslims, CNN reported.
The judge said Breivik accused the Labor Party of "treason" for advocating multiculturalism, the judge said. The island was the site of a party youth retreat.
Police refused to discuss their investigation into the possibility that two cells helped Breivik carry out the attacks Friday, saying the court hearing was closed so information wouldn't be disclosed.
Authorities lowered the death toll -- originally reported at 93 -- to 76, announcing Monday eight people were confirmed dead in the downtown bombing and 68 in the shooting on Utoya Island, CNN said. Police said they were searching the island to "make sure there are no casualties left."
Officials said at least 96 people were injured.
During his court hearing Monday, a court official told CNN Breivik seemed calm and was concise when trying to explain his actions.
"But when he started reading from his manifesto, he was stopped," the court official said.
The judge said Breivik wanted to save Europe from "cultural Marxism" and Muslim immigration, the British newspaper The Daily Telegraph reported. He admitted carrying out the acts, but did not plead guilty.
Breivik's goal wasn't to kill as many people as possible, but to send a "signal" to the Labor Party as a punishment for its "treason" of allowing immigration and to limit future recruitment to the party.
Heger ordered Breivik to remain in custody for eight weeks, until his next scheduled court appearance, as authorities investigate the case. He will be in isolation for at least four weeks, having access only to his attorney and will be barred from receiving letters or news reports.
Breivik, 32, said he is a member of Knights Templar, an armed Christian group fighting to rid the West of Islamic suppression. Officials are investigating a 1,500 page manifesto that was posted online just before the attacks Friday.
A moment of silence was observed across Norway, with the Norwegian government ordering trains halted as part of the observance.
Geir Lippestad, Breivik's attorney, said his client requested the hearing be in open court and he be allowed to wear the Knights Templar uniform.
"He has two wishes: the first is that there is a public hearing and the second is that he is allowed to wear a uniform," Lippestad said. Both were denied.
"He has said that he believed the actions were atrocious, but that in his head they were necessary," Lippestad said during a weekend interview on Norwegian television.
Acting National Police Chief Sveinung Sponheim said Sunday there has been "no progress" in determining Breivik's motives.
Investigators will conduct autopsies during the next few days, Sponheim said.
Among those killed on the island was Trond Berntsen, stepbrother of Crown Princess Mette-Marit, a statement released by the Royal House Communications Office indicated.