Breivik, clad in a dark suit, raised his fist as he entered the courtroom, presumably in support of his stance.
"I don't recognize Norwegian courts because you get your mandate from the Norwegian political parties who support multiculturalism," Breivik, 33, told the court. "I admit to the acts, but not criminal guilt."
Breivik, a right-wing fanatic, said that the killings -- 60 of which occurred at a youth camp on Norway's Utoya island -- were necessary to prevent Muslims from overtaking Norway, saying the country's liberal politicians and lax immigration policies essentially left him with no choice but to perpetrate the massacre.
To provide background on Breivik's extreme politics, prosecutors showed an anti-Muslim YouTube video he'd made prior to the attacks.
Breivik's trial is slated to last 10 weeks, and in that time the court will determine whether he'll be sent to prison or a mental hospital. If he's declared mentally competent to stand trial, he faces a maximum prison sentence of 21 years.
With more than 70 killed and 151 injured, the attack is the deadliest by a single gunman in recorded history.