Anders Breivik says he was acting in self-defense. He claims the attacks were necessary to stop Muslims from taking over Norway.
"I admit to the acts, but not criminal guilt," he told the court. He also said he does not recognize Norwegian courts because the political parties they receive mandates from support multiculturalism.
Breivik's Oslo bombing killed eight people. He also shot 69 others at a youth camp outside the city.
Breivik's mental health is a key issue during the trial. In an attempt to prove he is not insane, Breivik will call right-wing extremists and radical Islamists to testify during the trial, to show that others also share his view of clashing civilizations.
The Associated Press contributed to this report.