Norwegian mass murderer Anders Behring Breivik, who is serving a 21-year sentence for killing 77 people in a bomb and gun rampage last year, has complained that he is being held in inhumane conditions and is being denied freedom of expression, his lawyer said Friday.

"He has written a long complaint that he is being held in a section with particularly high security," Tord Jordet told The Associated Press. "He is today the only one in this ward and the security regime is the strictest in Norway."

The 33-year-old confessed killer has said the attacks were justified because his victims were traitors for embracing multiculturalism. The court found Breivik sane and said his sentence can be extended for as long as he is considered a danger to society.

In his letter to prison officials Breivik protested that the censorship of his letters was so strict that his freedom of expression was being impinged upon, Jordet said.

Norwegian tabloid VG, which said it had acquired a copy of the letter, quoted Breivik as saying he was allowed to use only a soft and bendable safety pen described by its manufacturer as "stab-resistant" because it yields at the slightest pressure and cannot be used as a weapon. Breivik was seen making avid notes with it during his 10-week trial at the Oslo District Court that ended in August.

Norwegian mass murderer Anders Behring Breivik uses a specially made soft pen, which is unable to be used as a weapon, in April during his trial in Oslo. (Heiko Junge/Associated Press)

He has said he wants to write books in prison, but claims the special pen cramps his hand, describing it as "an almost indescribable manifestation of sadism," VG reported.

Prison officials would not comment on the letter as they were still considering the complaint.

Ellen Bjercke, a spokeswoman for Ila Prison where Breivik is being held, said that Breivik was given an electric typewriter on Friday but that it was not connected to his letter of complaint. It was not clear when Breivik's letter was delivered to prison officials.

During his pre-trial detention Breivik was allowed a computer that could not be connected to the internet, but it was taken away from him when he started serving his sentence.

The Oslo District Court found Breivik guilty of terrorism and premeditated murder for the twin attacks on July 22, 2011, and gave him a 21-year prison sentence that can be extended if he's considered a threat.

Breivik confessed to setting off a bomb that ripped through Oslo's government district, killing eight people, then opening fire at the summer camp of the governing Labor Party's youth wing. Sixty-nine people died in the mayhem at the Utoya island camp outside the Norwegian capital Oslo before Breivik surrendered to a SWAT team.

The self-styled anti-Muslim militant denied criminal guilt, saying he's a commander of a resistance movement aiming to overthrow European governments and replace them with "patriotic" regimes that will deport Muslim immigrants. Police said they found no evidence of Breivik belonging to any such group.