Story highlights "Memories of goodness and joy cannot be erased," Prime Minister Jens Stoltenberg says

Anders Behring Breivik admits killing 77 people, including 69 at a youth camp

A court must determine whether he was sane at the time of the killings

The nation marks a moment of silence at the exact time the bomb detonated in Oslo

A year after Anders Behring Breivik killed 77 people in a bomb and gun rampage in Norway, Prime Minister Jens Stoltenberg urged Norwegians to "honor the dead by celebrating life."

Breivik, a self-declared ultranationalist who raged against Muslim immigration and multiculturalism, failed to achieve his goals, Stoltenberg said Sunday.

"The bombs and bullets were intended to change Norway. The Norwegian people responded by reasserting our values," Stoltenberg said. "The perpetrator failed to achieve what he set out to do. The people triumphed."

He called on people to remember their murdered loved ones with joy.

"No one can take the warm memories away from you. A father's reassuring hand. The sounds from a child's playroom. The memories of goodness and joy cannot be erased," he said.

And he asked Norwegians to respond to the horror by reaching out to people who may be suffering and in need of help.

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"We will not forget you when the long days of summer give way to autumn darkness," he said at a ceremony marking the anniversary. "Reach out. Show that you care. A chat about everyday things could help someone regain their will to live."

Norwegians observed a minute of silence to mark the worst atrocity on Norwegian soil since World War II, as Americans reeled from an enormous mass shooting two days earlier.

James E. Holmes , 24, is accused of killing 12 people and wounding 58 in the suburban Denver community of Aurora during a midnight screening of the Batman movie "The Dark Knight Rises" on Friday.

In Norway, Breivik is on trial on charges of voluntary homicide and committing acts of terror in the July 22, 2011 attacks.

He admits bombing a government building in Oslo before heading to an island summer camp, where he opened fire as terrified youth jumped into water to escape the hail of bullets.

The twin attacks killed 77 people, including 69 at the Labour Party summer camp. Many of the victims there were teenagers.

The court must determine whether Breivik was sane at the time of the killings. Mental health experts have given conflicting opinions.

Breivik gave chilling details of the gun rampage during his trial this year.

Without apparent emotion, he recounted firing more bullets into teenagers who were injured and couldn't escape, killing those who tried to "play dead" and driving others into the sea to drown.

Some survivors and relatives of victims in the courtroom wept as they listened to his detailed account of the attack on the youth camp on Utoya Island.

Breivik told the court he had made use of lessons learned from al Qaeda in planning his attacks, and was inspired by the Oklahoma City and World Trade Center bombings. He boasts of being an ultranationalist who killed his victims to fight multiculturalism in Norway, and says he acted out of "necessity."

Experts' assessment of Breivik's sanity will be a factor in determining what punishment he receives if convicted. Sentencing options could include imprisonment or confining him to a mental facility.

His verdict is expected August 24.

Numerous events are scheduled nationwide Sunday to mark the first anniversary of the killings, including religious services and wreath-laying ceremonies to pay tribute to victims of the attacks.