Counter-terrorism officers detained Abu Nusaybah outside BBC headquarters, The Guardian reported. Nusaybah had been at the BBC taping an appearance on "Newsnight."
In the interview, Nusaybah said Adebolajo had been tortured and sexually assaulted after he was arrested while studying in Kenya. He suggested that drove him to become more extreme in his religious views.
"He wasn't his bubbly self," Nusaybah said.
Adebolajo and another man were arrested Wednesday after a soldier, Lee Rigby, was almost decapitated outside an Army base in London's Woolwich district. Rigby was apparently targeted because he was wearing a T-shirt for a soldiers' charity.
Nusaybah said he and Adebolajo became friends when they were both living in Romford, Essex, in 2002. He said they converted to Islam in 2004 independent of each other.
"One day we bumped into each other. You didn't see a lot of black guys down there, so we talked and swapped numbers."
The Metropolitan Police refused to give details about why Nusaybah was arrested under the Terrorism Act of 2000. They said he is not a suspect in Rigby's killing.
"He was not arrested because of his comments on Newsnight," police said in a statement.
Sources told The Guardian Nusaybah has links to former members of al-Muhajiroun, an organization banned in Britain.