Anders Behring Breivik awaited his sentencing in an Oslo court on Friday.

A court on Friday sentenced Anders Behring Breivik, the Norwegian extremist who admitted killing 77 people, to at least 21 years in prison after ruling that he was sane when he carried out his country’s worst peacetime atrocity. The sentence was the most severe permitted under Norwegian law, but it can be extended at a later date if he is still deemed to be a danger to society.

Mr. Breivik, 33, who had insisted that he was sane when he carried out the attacks last year as part of what he called a campaign against multiculturalism in Norway, smiled when the verdict was announced. As he arrived in court on Friday, lightly bearded and wearing a dark suit and tie, he flashed a right-wing salute with his right arm jutting from his body and his fist clenched.

His 10-week trial ended in June. Defense lawyers argued that Mr. Breivik was sane when he bombed buildings in downtown Oslo, killing eight people, before killing 69 people at a summer youth camp run by the Labour Party on Utoya island, and should therefore be sentenced to prison. Prosecutors said that he was mentally ill, was not criminally responsible, and should be hospitalized instead. It was not immediately clear whether prosecutors planned an appeal.

Experts said they were not aware of any previous case in Norwegian legal history in which prosecutors had called for an insanity verdict and defense lawyers had advocated conviction.

Judge Wenche Elizabeth Arntzen said on Friday that the decision reached by the five-member panel hearing the case had been unanimous. Reading from a 90-page judgment, she refuted Mr. Breivik’s assertion that he acted as part of a network called the Knights Templar, saying there was no evidence to prove its existence. Mr. Breivik has said he was present when it was founded in London in 2002.

Labour Party supporters in court on Friday hugged as the verdict was announced.Mr. Breivik is to be incarcerated in isolation at Ila prison on the outskirts of Oslo, news reports said, in a three-room cell with an exercise area, a television set and a laptop computer that is not connected to the Internet.

Some relatives of the dead welcomed the verdict. “Now we won’t hear about him for quite a while. Now we can have peace and quiet,” Per Balch Soerensen, whose daughter was among those killed in the shooting massacre, told Denmarks TV2, according to The Associated Press. “He doesn’t mean anything to me, he is just air.”

Mr. Breivik had never denied carrying out the killings and his trial revolved around the question of his sanity at the time of the attacks. The bombing and shooting spree convulsed Norway, and the country’s police chief was forced to resign this month after an independent commission found that the police could have averted or at least disrupted Mr. Breivik’s plot.

The inquest by the panel, the July 22 Commission, named after the date of the massacre in 2011, said the police had failed in their duty to protect the youth camp on Utoya Island. Most of the victim were teenagers..

The panel’s 500-page report also faulted the police in Oslo, where hours before the shooting spree, Mr. Breivik had parked a van packed with explosives near government buildings. He was seen in a getaway car, which he drove to the island, but police officers failed to share a description of the vehicle.

The report chronicled an array of errors and blunders at nearly every level of law enforcement in Norway, a country that was traumatized by the scale and audacity of the attacks. The trial offered a platform to a man whose views repulsed most Norwegians.

Mr. Breivik’s trial underscored the role of psychiatry in the country’s legal system and prompted calls for a review of the balance between insanity and guilt.

“It is a reverse situation, since they want him acquitted” by reason of insanity, Geir Lippestad, one of Mr. Breivik’s lawyers, said as the trial drew to a close in June, gesturing to the prosecutors on the opposite bench. “I say that their plea should not be accepted, and Anders Behring Breivik should be treated as leniently as possible.”

Under Norwegian law, if a defendant was psychotic at the time of his crime, he cannot be punished. Mr. Breivik has been the subject of two conflicting psychiatric reports, one saying that he was a psychotic paranoid schizophrenic, the second that he had narcissistic and antisocial personality disorders, but was legally competent.

On the final day of his trial in June, as Mr. Breivik gave a statement, around 20 survivors and family members filed out of the courtroom in protest.

In an hourlong, rambling warning about the evils of Norwegian multiculturalism, Mr. Breivik said: “I acted in the principle of necessity for my country, so I ask to be acquitted.”

Alf Petter Hogberg, a professor of public and international law at Oslo University, said before the verdict that he thought an appeal be made, with a hearing in the Norwegian Supreme Court likely as early as November. “If the judges decide he is sane then I am sure the prosecutors will appeal. If there is a 5 percent doubt, then he should be considered insane. So they should appeal,” he said.