A friend of one of the suspected killers of a soldier butchered in a London street was arrested by counter-terrorism officers yesterday after going on television to allege that Michael Adebolajo was tortured in Kenya and harassed by MI5 – who asked him to spy for them.

The allegations came from Abu Nusaybah, who was arrested under terrorism legislation minutes after finishing recording his interview for BBC Newsnight.

The Guardian understands that Nusaybah is connected to extremists formerly in the banned group al-Muhajiroun. Counter-terrorism officers turned up at reception at the BBC's central London headquarters as his interview was being recorded an hour before it was broadcast. The officers waited, then arrested Nusaybah as he left the BBC's headquarters.

In the interview, Abu Nusaybah in part attributed Adebolajo's radicalisation to his alleged ill-treatment in Kenya.

Nusaybah alleged that Adebolajo was arrested while studying in a village in Kenya last year. After refusing to answer questions, Adebolajo was told that he was "not in the UK" and was then, he claimed, sexually assaulted. On his return, "he became more reclined [sic], less talkative. He wasn't his bubbly self," said Nusaybah, who said that the experience further radicalised Adebolajo.

The Guardian has learned that, last year, Adebolajo went to a lawyer to complain of harassment by MI5, who are drawing criticism today for knowing about the 28-year-old but not assessing him to be a bigger terrorist threat.

He was arrested at the scene of the soldier's murder in Woolwich on Wednesday in the first terror-related death on mainland Britain since the 7 July 2005 bombings.

The Guardian also understands from Whitehall sources that they were aware that Adebolajo was detained in Kenya before being deported.

Nusaybah said he had known Adebolajo since they grew up together in Romford, Essex.

Since the attack on the soldier, Lee Rigby, on Wednesday, a total of six people have been arrested. Nusaybah's was the first under counter-terrorism laws; the rest have been detained under regular criminal laws. Nusaybah was arrested on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.

Asked about the unusual sequence of events leading to the arrest, the Metropolitan police said: "He was not arrested because of his comments on Newsnight."

The police would not say what led to his arrest, which BBC insiders said had left them shocked. Police also said he was not arrested in connection to the murder of Lee Rigby. Nusaybah's home is understood to be one of two addresses in east London being searched by officers after his arrest.

In the interview, Abu Nusaybah described how the two men became friends after meeting in Romford in 2002, before both had converted to Islam. "I used to go there to chill out with friends," he said. "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 two men converted to Islam independently in 2004. "I came into Islam in 2004 and he came in four months later," Nusaybah said.

He also confirmed that Adebolajo frequented meetings of the now-banned al-Muhajiroun group, where he listened to the preachings of extremist clerics. "He attended some of their activities, but he was an independent guy. He would float about," Nusaybah said.

Adebolajo did not want to see sharia law imposed in Britain, but thought it more sensible for someone like him to go and live in a Muslim country, according to the interviewee. "He wanted to be qualified to teach and to do fitness training. He could use that to go abroad and live in a Muslim country."

Abu Nusaybah said that he believed that Adebolajo became radicalised about six months ago. He said he saw profound changes in Adebolajo's character at that time, which he attributed to his experiences in Kenya and to events on his return to Britain. "I believe that certain events that happened to him recently had an impact in shaping the changes. He became more reclined [sic], less talkative. He wasn't his bubbly self," he said.

Adebolajo had told Nusaybah how he had gone to study in a village in Kenya when he and others were rounded up by the Kenyan army. When he was interrogated, he refused to speak. "They told him, 'You are not in the UK now.' They took his private parts and said, 'We will F you.' He told me he was physically assaulted and sexually threatened. If you looked at his face, he was holding back tears," Nusaybah said.

When Adebolajo returned to England, he was interviewed by MI5 officers and, according to Nusaybah, was planning to leave again to live in a Muslim country. "His whole concept was he wanted to live in a Muslim land because at the time he was being harassed by MI5.

"They were knocking and knocking on his door. He pretended not to be there, but then he spoke to the agent. They said, 'We just want to speak to you.' They wanted to ask him if he knew certain people," he said.

"But after him saying that he didn't know these individuals, he said they asked him if he would be interested in working for them. He was explicit in that he refused to work for them but he did confirm he didn't know the individuals."

"His word was, 'They are bugging me,'" said Nusaybah.

In a statement about the arrest, Scotland Yard said: "At around 2130 hours 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."