He held up a sign before being seated on Thursday (January 20).
Breivik, a far-right extremist, killed 77 people in Norway's worst peacetime atrocity in July 2011. He killed eight with a car bomb in Oslo and then gunned down 69, most of them teenagers, at a Labour Party youth camp.
The 42-year-old neo-Nazi is serving Norway's maximum sentence of 21 years, which can be extended indefinitely if he is deemed a continued threat to society. But after a decade in prison he is entitled to apply for parole.