Abu Nusaybah, a friend of Woolwich attack suspect Michael Adebolajo was arrested at BBC studios after giving an interview claiming MI5 tried to recruit the fanatic.
The Metropolitan Police confirmed that a 31-year-old man had been arrested under the terrorism act and was being questioned. Two adressed in east London were being raided, they said.
The Muslim, who has known Adebolajo since they met in Romford in 2002 before they both converted to the faith, has been vocal about the Woolwich attack on social networking sites. .
He has made a number of unverified accusations, including that six months ago Adebolajo told him he was tortured and sexually assaulted by Kenyan troops when he travelled to the country and refused to answer their questions.
When asked how he could have changed from the “gentle” man he described into one capable of carrying out the attacks, Mr Nusaybah told Newsnight: “I believe that these certain events that happened to him recently kind of had an impact in shaping that change.
“That change wasn’t necessarily one that became overtly aggressive or anything like that, he just became more reclined and less talkative and so forth, he wasn’t as bubbly… It was almost as if he was somewhere else.”
Mr Nusaybah said that when the footage of the attack emerged he thought it was a “joke”, that it “didn’t make sense” because Adebolajo simply wanted to go and live in Muslim lands and get away from his problems.
One of Adebolajo’s main issues was that he after his return from Kenya he was being “hassled” by MI5 who kept knocking on his door, Mr Nusaybah said.
“Initially they wanted to ask him whether he knew certain individuals, but on him saying that he did know these individuals they asked him whether he would be interested in working for them.”
He was “explicit” in his refusal to join the security services, Mr Nusaybah added.
Before the footage was played the interviewer, Richard Watson, told the viewers: “He (Nusaybah) came into the BBC, we conducted the interview, and at the end of the interview I am told that three special branch were on BBC premises and arrested the man.
“Very dramatic events tonight.”
A Scotland Yard spokesperson said: “At around 21.30 on Friday, 24 May, a 31-year-old man was arrested in London by officers from the MPS Counter Terrorism Command on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.
“The man has been taken to a south London police station, where he remains in custody. Search warrants are being executed at two residential addresses in east London.”