Confessed mass killer Anders Breivik says he was planning to capture and decapitate former Norwegian Prime Minister Gro Harlem Brundtland during his shooting massacre on Utoya island.

Breivik told a court that he planned to film the beheading and post the video on the internet.

Brundtland had already left the Labour Party’s youth camp on Utoya when Breivik arrived on July 22.

The far-right fanatic said he also planned to kill many more than the 69 people who died on the island.

There were nearly 600 people there. Breivik said, “the goal was to kill them all”.

Earlier he told a court he planned to bomb Oslo's government district and the Labour Party's office and a third target.

The three bombs would be followed by several shooting massacres, if he survived, he said.

He decided against multiple bombs because building one was "much more difficult than I thought".

Breivik has confessed to the bomb and shooting rampage, but rejects criminal guilt saying he was acting to protect Norway and Europe.

"There would be three car bombs, followed by a firearm-based action," Breivik told the Oslo court.

A total of 77 people were killed in his car bomb and shooting attacks.

Questioned by prosecutors, Breivik said his original plan was to build three bombs. One would be placed at the government district and the second at the office of the governing Labour Party.

He had several options for the third target.

"I settled on the palace in a setting where the royal family wouldn't be hurt," he said. "Most nationalists and cultural conservatives are supporters of the monarchy, including myself."