Prosecutors said today there are enough doubts about confessed mass killer Anders Behring Breivik's sanity that he should be committed to compulsory psychiatric care instead of prison.

If the court comes to the same conclusion when it issues its ruling, expected next month, it would mean that Breivik will avoid criminal responsibility for Norway's worst peacetime massacre.

The attacks at Norway's government headquarters and a youth summer camp would then not be considered acts of political terrorism, but the work of a blood-thirsty madman.

"We request that he is transferred to compulsory psychiatric care," prosecutor Svein Holden told the court in closing arguments.

The defence is likely to refute the insanity finding on Friday, the last day of the 10-week trial.

Breivik, who styles himself as an anti-Muslim militant, claims he is sane and that his attacks were motivated by his political views.

Just like when the trial started in mid-April, the 33-year-old Norwegian flashed a clenched-fist salute with his right arm before he was led out of the court on Thursday.