Greg Johnson was detained under immigration law on the basis that he posed a threat to national interests, according to the police security agency PST.
“He stands for and communicates an extreme right-wing ideology,” PST spokesman Martin Bernsen said. “There’s a danger that it can result in violence.”
The authorities are now seeking to deport him back to the US later this week.
Mr Johnson, who runs the alt-right organisation Counter-Currents Publishing, was scheduled to take part in the Scandza Forum, a far-right network known for its anti-Semitic and racist views.
He has called for white-only states and once claimed that ‘”excessive kindness” was “Hitler’s greatest flaw”, according to the anti-racism campaign group HOPE not hate.
Before heading to the conference in Oslo, he complained that the Norwegian website Filter Nyheter was “agitating to stop my entry” by claiming he had expressed support for the far-right terrorist Anders Breivik.
“I have always consistently condemned violence and terrorism,” he wrote on his website, before suggesting that his writings about Breivik had been “taken out of context”.
He added: “In fact, it was a text that discussed Breivik’s own rationale for the crime."
One of the other speakers at the event was listed as Edward Dutton, a British academic who has written about racial differences.
Mr Bernsen said that the PST was working “to keep this milieu as small as possible”. He told Aftenposten: “It’s important to stop those trying to radicalise others to a more extreme direction.”
Around 50 demonstrators protested against the Scandza Forum conference in the Norwegian capital on Saturday, chanting: “No racism in our streets.”
Additional reporting by Associated Press