Should the Oslo district court judges however find that the confessed killer is sane, prosecutor Svein Holden asked them to sentence Breivik to Norway's maximum penalty of 21 years in prison, which can be extended for as long as he is considered a danger to society.
A visibly angry Breivik stood up and made a fascist-style stiff arm salute when he heard the call for psychiatric confinement.
"Our request is that he be obliged to undergo psychiatric treatment" in a closed unit, Holden said, wrapping up the prosecution's three-hour closing arguments on the next-to-last day of the trial.
"If the court concludes ... that Breivik is responsible, the prosecution deems that the conditions have been fulfilled for prison" with a provision allowing the sentence to be extended, he added.
In such case, Holden said, "there is no doubt that Breivik must receive the harshest penalty: 21 years" with the extension provision.
Following Holden's remarks, the 33-year-old right-wing extremist stood up and touched a clenched right fist to his chest before stretching his arm out in a nationalist salute he had made on the first days of his trial in April but had stopped doing at the request of his lawyers.
Breivik is intent on being found sane to ensure his Islamophobic ideology is not written off as crazy ravings.
The five judges are expected to announce their verdict on either July 20th or August 24th. The trial is to wrap up on Friday with the defence's closing arguments.
Laying out the prosecution's arguments, Holden said he would rather err on the side of caution, stressing it was worse to sentence someone who is psychotic to prison than to send someone who is not psychotic to psychiatric care.
"We are not convinced or sure that Breivik is criminally insane, but we are in doubt," Holden said.
Psychiatric evaluations of Breivik's mental health have sharply contradicted each other.
But Holden and his colleague Inga Bejer Engh followed the line presented by a first court-ordered assessment, which found Breivik to be suffering from paranoid schizophrenia.
Holden pointed for instance to his stubborn claims about a militant nationalist network, the Knights Templar, despite "overwhelming evidence" that it did not exist.
Earlier, Bejer Engh told the court that "July 22nd, 2011 was and is still a national trauma. Thousands of people were directly or indirectly affected."
She cautioned that many of the most pressing questions in the case, including how "a citizen became a murder machine", probably would not be sufficiently answered to satisfy those affected.
On July 22nd, Breivik first set off a car bomb outside government buildings in Oslo, killing eight people, before travelling to Utøya island, north-west of the capital, where he spent more than an hour methodically shooting and killing another 69 people, mostly teenagers.
The victims had been attending a summer camp hosted by the governing Labour Party's youth organisation.
"I find it difficult to find words to describe the 80 minutes he spent ... with only one goal: to kill as many people as possible," Bejer Engh said.
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Breivik, who has been charged with "acts of terror", has told the court what he did was "cruel but necessary" to protect Norway from a wave of multiculturalism and a "Muslim invasion".
A majority of psychiatric experts who have observed Breivik in detention and in court have agreed that he is not suffering from a psychosis, but rather some form of personality disorder -- which would mean he could be sentenced to prison.
An opinion poll published Thursday indicated 74 percent of Norwegians want Breivik to be sent to prison, while only 10 percent think he should be sent to an insane asylum.
Breivik himself has said court-ordered psychiatric confinement would be "a fate worse than death."
The trial, which opened on April 16th, concludes on Friday with the defence lawyers' closing arguments. They are expected to call for Breivik to be acquitted, since, despite his confession, he has pleaded not guilty.
Alternatively, they will ask that he be found sane and sent to prison.