Norwegian King Harald and Queen Sonja attended a solemn service at the city's main Lutheran cathedral along with Prime Minister Jens Stoltenberg to mourn the dead from Friday's bombing and shooting spree. The death toll Sunday rose to 93, CNN reported.
At least 96 others were injured in the attacks that began with a car bombing of the prime minister's offices and a 90-minute shooting spree in the midst of a Labor Party youth rally on the nearby island of Utoeya, the BBC said.
Police arrested Anders Behring Breivik, 32, on the island. His lawyer, Geir Lippestad, told media his client surrendered when he ran out of ammunition.
Lippestad said his client told police he was responsible for the attacks, but didn't claim any guilt, The New York Times said.
"He has said that he believed the actions were atrocious, but that in his head they were necessary," the lawyer said.
Breivik was to make his first court appearance Monday.
Meanwhile, police were poring over a 1,500-page manifesto purportedly written and posted on the Internet by Breivik soon before the attacks.
The author said his goal was to "seize political and military control of Western European countries and implement a cultural conservative political agenda," The Times said.
Indications were that the attacker acted alone, although investigators said they were still looking for accomplices.