Britain's intelligence agencies came under fresh scrutiny on Saturday following claims that MI5 had tried to recruit one of two Islamists accused of butchering a soldier in London.
Abu Nusaybah, who says he is a childhood friend of murder suspect Michael Adebolajo, was arrested by counter-terrorism police shortly after making the claims on BBC television on Friday night.
His comments come amid community tensions following the murder of 25-year-old Lee Rigby outside a barracks in Woolwich, southeast London, on Wednesday afternoon by two men spouting Islamist rants.
An inter-faith group reported a large spike in anti-Muslim incidents, while up to 2,000 members of the far-right English Defence League (EDL) staged a protest in the northeastern city of Newcastle.
Adebolajo, 28, was captured on film after the murder brandishing a bloodied knife and meat cleaver and claiming he had killed the soldier because British forces killed Muslims every day.
He had reportedly tried and failed to previously travel to Somalia to fight alongside Al-Qaeda-linked Shebab insurgents.
Abu Nasaybah said his friend had recently travelled to Kenya, where he said he was rounded up and physically and sexually abused by local security services.
On his return Adebolajo appeared changed, his friend said, while he had also attracted the attention of Britain's security services, who called at his home several times.
He was "basically being harassed by MI5, this is something that he specifically mentioned to me" when they spoke six months ago, Abu Nusaybah said.
He added: "He was explicit in that he refused to work for them but he did confirm that he didn't know the individuals" they were asking about.
Adebolajo and his alleged co-conspirator Michael Adebowale, 22, remain under armed guard in separate hospitals after being shot by police during their arrest at the scene of the grisly murder.
Faith Matters, a state-funded organisation which works to reduce extremism, said it has recorded a huge increase in anti-Muslim incidents reported to its helpline since the attack.
"It's a hugely worrying development," director Fiyaz Mughal told AFP, saying the organisation had been informed of 162 incidents in the past 48 hours, compared to a daily average of four to six.
They were mainly verbal attacks on women wearing the Islamic headscarf in the street, he said, but there were also online attacks and some violence.
A woman in her 50s had been punched unconscious on the outskirts of Oldham, near Manchester in northwest England, while two mosques had been attacked in the south of the country.
Between 1,500 and 2,000 members of the anti-Islam EDL gathered in Newcastle on Saturday afternoon, police said, for a long-planned rally given added impetus by the soldier's murder.
They sang "RIP Lee Rigby" and chanted "Whose streets? Our streets", while some people waved British flags.
Up to 400 of their opponents staged a counter-protest, but the day passed off without major incident.
A small impromptu protest also took place in Manchester, northwest England, while another demonstration is planned for outside Prime Minister David Cameron's Downing Street office on Monday.
Both murder suspects were known to the intelligence services and Adebolajo had links to the banned radical Al-Muhajiroun movement, but ministers and security experts have warned of the difficulty in keeping track of everybody with extreme views.
In his BBC interview, Abu Nusaybah admitted he had toyed with radical Islam himself but was appalled at his friend's actions.
He was arrested at the BBC on suspicion of "commission, preparation or instigation of acts of terrorism", police said. The arrest is not thought to be linked directly to Wednesday's murder.
The backgrounds of the two murder suspects are remarkably similar.
Both were brought up by Nigerian Christians and converted to Islam in their teens, and recently were seen handing out extremist literature in the streets -- to the concern of their families.
Adebowale's mother, Juliet Obasuyi, sought help from friends and an imam after growing increasingly concerned about her son, particularly when he dropped out of university nine months ago.
"He is from a strong Christian family but he is turning to Islam and turning against the family," she was quoted in newspapers as telling a Muslim neighbour.
"He needs spiritual guidance before he radicalises himself."
The University of Greenwich, near Woolwich, meanwhile confirmed Adebolajo was a student for two years but said his work was "unsatisfactory" and he was not permitted to finish his course.
Vice-chancellor David Maguire said the university had launched an investigation into any potential links to extremism.