Norway mass killer Anders Behring Breivik will not appeal an Oslo court's verdict on Friday finding him sane and sending him to prison for 21 years, his lawyer said after the conviction was handed down.

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"He says he won't appeal now that he has been found sane," Geir Lippestad told journalists outside the courtroom during a break in the proceedings.

The 33-year-old far-right extremist wanted to be found sane so that his Islamophobic and anti-multicultural ideology would not be seen as the rantings of a crazy person.

Breivik smiled on Friday as the verdict and sentence were read out.

During his 10-week trial, he had confessed to the attacks, seeing himself as a Nordic warrior against Europe's "Muslim invasion" and all those who promote multiculturalism.

The main question the court had to determine was whether he was sane and could be held responsible for his actions.

"He said the verdict was not a surprise," another of Breivik's lawyers, Odd Ivar Grøn, told the online version of tabloid Verdens Gang during the break.

Breivik is expected to serve his sentence at the high-security Ila prison near Oslo.

The sentence can be extended indefinitely as long as Breivik is considered a threat to society.

The prosecution, which had called for Breivik to be ordered to undergo psychiatric care on the basis of a medical evaluation that found him to be suffering from paranoid schizophrenia, has the option to appeal the verdict.