Oslo, Norway (CNN) -- Anders Behring Breivik, who admits killing 77 people in Norway last summer, was trying to kill the prime minister and other government ministers by bombing a building in Oslo, he testified Thursday.
The blast killed eight people, but Breivik said he considered it a failure because the building did not collapse.
Breivik is on trial on charges of voluntary homicide and committing acts of terror in the July 22 attacks. He admits carrying out the Oslo bombing and then shooting 69 people dead on nearby Utoya Island.
He boasts of being an ultranationalist who killed his victims to fight multiculturalism in Norway.
He decided to carry out a gun attack on a Labour Party youth camp on the island of Utoya after his initial plan to target a journalists' conference did not work out, he said.
He also hoped to kill former Prime Minister Gro Harlem Brundtland and decapitate her with a knife or bayonet, he said. He planned to film the murder on his iPhone and upload the video to the Internet, he said.
He concentrated on killing people over the age of 18 on the island, he said, because he thought the killings of younger people would be harshly criticized.
Many of the victims were younger than 18, but he said he did not regret what he did.
"I would do it again," he said.
His attorney, Geir Lippestad, warned in a news conference after the session that Friday -- when testimony will focus on how Breivik carried out the killings on Utoya -- "is going to be the toughest day."
There was utter silence in the court Thursday as Breivik testified, marked only by the sound of crying from some members of the public.
He used the video game "Modern Warfare 2" as training for his shooting spree, he testified. Players of the game, one of the "Call of Duty" series, work together as soldiers to shoot opponents.
Breivik also went through a period of playing the online fantasy game "World of Warcraft" up to 16 hours a day, he testified.
He initially planned to carry out three bomb attacks followed by a "gun-based action," he told the court, but eventually was only able to make enough explosives for one car bomb.
Asked about his intended bombing targets, Breivik said: "The first two were clear: the government quarter and the Labour Party headquarters. The third, I was not sure."
He said he at first had reservations about hitting the Labour Party headquarters because of the civilians and innocent people in offices around the building.
"But then I thought: There are not many better targets in Norway," he said.
Breivik said he had also considered bombing the Royal Palace -- but without harming the royal family -- City Hall and Aftenposten, a national newspaper.
Breivik joked about the psychiatrists who have labeled him insane and shrugged off a question about how he thinks victims' relatives feel when they see him smile in court.
"I think they react in a natural way," he said.
As the hearing concluded, prosecutor Inga Bejer Engh asked Breivik: "How is your empathy?"
Breivik did not answer but asked to have the question put to him again Friday, saying it was important.
Lippestad is also expected to tell the court Friday about what were the sources for Breivik's information.
His client did not raise his fist in salute when he entered the court Thursday, respecting a request from the families of his victims.
The relatives were upset that he had been making the gesture each morning, Lippestad said Wednesday.
Breivik said in court Wednesday that he should either get the death penalty or be acquitted, ridiculing the idea that he would be sent to prison or a mental hospital for his actions. Norway does not have the death penalty.
The trial is expected to last up to 10 weeks.
Breivik boasted Tuesday that he had carried out "the most sophisticated and spectacular political attack in Europe since World War II" when he went on his gun-and-bomb rampage.
He planned his killings as a suicide attack, he said.
"I didn't expect to survive that day," he said Tuesday.
Breivik testified Tuesday and Wednesday after declaring Monday that he had carried out the massacre but was not guilty because the killings had been necessary.
Lippestad said it was important to his client that people see him as sane.
"He thinks that it won't have any effect if he is considered insane," Lippestad said.
Experts have given different opinions about Breivik's sanity, which 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 testimony is not being broadcast due to a court ruling.
Most of the victims' relatives did not want Breivik's remarks televised, and presiding Judge Wenche Elizabeth Arntzen rejected Breivik's claim that airing it was a human right.
Court papers indicated the five judges hearing the case did not want the trial to become a platform for Breivik to air his political views, or for them to distract from the legal issues involved.
Breivik has said his rampage was meant to save Norway from being taken over by multicultural forces and to prevent ethnic cleansing of Norwegians.
In a 1,500-page manifesto attributed to him, Breivik railed against Muslim immigration and European liberalism -- including the ruling Labour Party, which he said was allowing the "Islamification of Europe."
Journalist Olav Mellingsaeter contributed to this report.