He's been active on anti-Islam websites.
A second person (armed with a knife) has been arrested in Sundvollen, where many victim's relatives are assembled in hotel.
(He said that he was a member of Social-Democratic youth and was armed only for self-defence, according to NRK)
They're going to arrest somebody for owning a knife after some one just killed 87 people with a machinegun? I mean, is that supposed to be competent police work or something like that?
Actually, I'm not sure if it's really a machinegun - the Norwegian word is "automatvåpen" ('automatic weapon').
As Norway struggles to come to terms with its greatest loss of life in decades, all eyes are on the man charged in the explosion in central Oslo and a deadly shooting rampage.
While police have not officially named him, Norwegian television and newspaper reports have identified the suspect as 32-year-old Anders Behring Breivik, of Norwegian origin.
A picture is emerging, gleaned from official sources and social media, of a right-wing Christian fundamentalist who may have had an issue with Norway's multi-cultural society.
Norwegian and international news outlets have run photographs of a blond man with blue-green eyes and chiseled features, dressed in a preppy style.
A victim who was shot during the attack on the island told CNN Saturday that he had seen pictures of Breivik taken from what is believed to be his Facebook page and shown on NRK and TV2. The victim said he recognized the man from the news reports as the gunman.
Breivik is a member of the Oslo Pistol Club and has three weapons registered in his name, according to leading Norwegian newspaper VG, citing Norway's official weapons register. They are a Glock pistol, a rifle and a shotgun, VG reported.
A post in Breivik's name on an online forum, Document.no, from December 2009, talks about young non-Muslim teenagers being "in an especially precarious situation with regards to being harassed by Islamic youth."
"I know of many hundred occasions where non-Muslims have been robbed, beaten up and harassed by Islamic gangs," the post reads. "I had a best friend between the ages of 12-17 who was a Pakistani, so I was one of the many protected, cool 'potatoes' that had protection. But this also made me see the hypocrisy up close and personal and made me nauseous."
Police have not ruled out the possibility that other people may have helped the suspect but say at present they are not pursuing any other arrests.
"The official questioning is starting now," Roger Andresen, a police official, told reporters during a news conference Saturday.
The suspect was cooperating with police, making it clear he wanted to explain himself, Andresen said.
Two addresses connected to the suspect are being searched, police said. One of them is believed to be an apartment in Oslo and the other a farm in Hedmark.
NRK reports that he is registered as having run a company which produced "vegetables, melons, roots and tubers" -- an industry which allows access to large amounts of fertilizer, the broadcaster notes, which can be used for explosives.
More details on the man are sure to emerge in the coming hours and days. But what many Norwegians find hard to comprehend is that the chief suspect appears to be one of their own.
Prime Minister Jens Stoltenberg stressed that whoever was behind the attacks must be dealt with properly.
"It is very important that those who are responsible -- one or several persons -- are sentenced according to Norwegian law, in the Norwegian system of justice," he said.
"Norway is a small country but it is a proud country. We are all very close, especially in times like this."
Sounds like they just arrested some bystander for possessing a knife...
In the safest, most boring country, the worst lone gunman shooting happens. The worst in the world, in history. But it will not make our country worse. The safe, boring democracy will supply him with a defense lawyer as is his right. He will not get more than 21 years in prison as is the maximum extent of the law. Our democracy does not allow for enough punishment to satisfy my need for revenge, as is its intention. We will not become worse, we will be better. We lived in a land where this is possible, even easy. And we will keep living in a land where this is possible, even easy. We are open, we are free and we are together. We are vulnerable by choice. And we will keep on like that, that's how we want to live. We will not be worse because of the worst. We must be good because of the best.
Another youth, Dana Barzingi, described the massacre to state broadcaster NRK. "It was a bloodbath. There was nowhere to flee, only the water, and when people tried to swim away he just shot them."
"He kept coming," said Barzingi. "He went around and around pretending to be a rescue worker and said people should come to get help. Then he shot them."