Norwegian prosecutors on Wednesday indicted Anders Behring Breivik on terror and murder charges for killing 77 people in a bomb and shooting rampage on 22 July 2011 but said the confessed mass killer was unlikely to go to prison for the country's worst peacetime massacre.

Prosecutors said they considered the 33-year-old rightwing extremist to be psychotic and would seek a sentence of involuntary commitment to psychiatric care instead of imprisonment unless new information about his mental health emerged during the trial, due to start in April.

As expected, they charged him under a paragraph in Norway's anti-terror law that refers to violent acts intended to disrupt key government functions or spread fears in the population.

Breivik has confessed to the attacks but denies criminal guilt, portraying the victims as "traitors" for embracing immigration policies he claims will result in an Islamic colonisation of Norway.

Eight people were killed when a bomb exploded in Oslo, and another 69 died in a shooting spree on Utøya island outside the capital, where the youth wing of the governing Labour party was holding its annual summer camp.

Reading from the indictment, prosecutor Inga Bejer Engh said 34 of the victims at Utoya were between 14 and 17 years old, 22 were aged 18-20, six were between 21 and 25 and seven were older than 25.

She said 67 died of gunshot wounds, and two died of fall injuries or drowning. In addition, 33 people were wounded by bullets.

Police spokesman Tore Jo Nielsen told the Norwegian broadcaster NRK that Breivik was totally calm when he was read the charges.

The terror charges carry a maximum penalty of 21 years in prison but prosecutors are working under the assumption that Breivik is legally insane and therefore unfit for prison. However, they said that this assessment could change during the trial.

A second, court-ordered psychiatric evaluation of Breivik is continuing after an initial review, which concluded he was a paranoid schizophrenic, met with widespread criticism. Some experts questioned whether someone with a grave mental illness would be capable of carrying out attacks requiring such meticulous preparation.