Mass murderer Anders Behring Breivik has apologised to "militant nationalists" for not killing more people during a shooting spree and bomb attack in Norway.

He was speaking after a five-judge panel in Oslo ruled he was sane as he killed 77 people last year and sentenced him to 21 years in jail.

Breivik - who claimed the killings were necessary to stop the "Islamisation" of the country - also said he did not accept the "illegitimate" judgement, but would not appeal the verdict.

"I wish to apologise to all militant nationalists that I wasn't able to execute more," he said before his remarks were cut off by the presiding judge.

As the judges convicted Breivik of terrorism and premeditated murder, sentencing him to 21 years in a high security prison, he smirked to the court.

Sky's Robert Nisbet, reporting from Oslo, said Breivik would probably spend the rest of his life behind bars with the justice system increasing his sentence in five year increments.

He added the declaration that Breivik was not insane was exactly what the killer wanted.

"He wanted to be viewed as a warrior against multiculturalism, not a madman," he said.

Nisbet said in its 90-page document on the judgement, the court said Breivik was narcissistic and had a personality disorder, but was not insane.

He was not aggressive or defensive when questioned by police which usually happens with psychotic defendants.

Lawyers for the 33-year-old right-wing extremist said before the decision that Breivik would appeal any insanity ruling, which he said would be "worse than death", but accept a prison sentence.

Outside Norway, the sentence was seen by many as mild. The maximum sentence allowed under Norwegian law is 21 years but such sentences can be extended as long as an inmate is considered too dangerous to be released.

As judges took turns reading sections of the ruling, people were crying inside the courtroom as they relived the horror of the killing spree. Many families of the victims and survivors were present.

Some who lost loved ones in the attacks welcomed the ruling.

"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 Denmark's TV2.

"He doesn't mean anything to me, he is just air."

Per Anders Langerod, a Utoya survivor, said: "I actually want to visit him in his jail cell and yell at him...really hard for 15 minutes, 20 minutes, throw some plates on the floor, you know, show anger."

Another survivor of the Utoya Island attack, Tore Bekkedal, said more important than the outcome of the trial was the way the whole process was conducted "in accordance with the standards of our society".

"Everyone has been determined to prove the strength of the values of our society, the very values he attacked."

He added: "We didn't want to grant him the satisfaction of seeing us abandon our principles and abandon what we believe in simply because he tried to tell us to."

Bjorn Kasper Ilaug, who rescued 24 youngsters from the island of Utoya, told Sky News he was relieved.

"I hoped for this. It means we can move forward and justice is done. I hope the Norwegian authorities do not appeal this verdict."

Sky News reporter Trygve Sorvaag, who was inside court, said the 40-day trial has cost £7m.

He added that the people of Norway are pleased with the verdict, which has strengthened their belief in the justice system. Tweets expressed how Norwegians could now move on after the harrowing massacre.

Breivik detonated a bomb outside the office of Prime Minister Jens Stoltenberg, killing eight people, and then drove to the island of Utoya where he shot dead 69 people.