Media playback is unsupported on your device Media caption Shaker Aamer told Victoria Derbyshire that he believes a British agent witnessed his beating in Afghanistan

The UK's last Guantanamo Bay detainee, who returned home in October after 14 years in captivity, has told extremists to "get the hell out" of the country.

Shaker Aamer told the Mail on Sunday that he denounced attacks such as the murder of Fusilier Lee Rigby, saying "you cannot just kill anybody".

Mr Aamer said reuniting with his wife in London had "washed away the pain".

But he told the BBC he feared his four children, including a son he had never met before, saw him as a stranger.

The 48-year-old was held at the US military base in Cuba over allegations he had led a Taliban unit in Afghanistan and met al-Qaeda leader Osama Bin Laden, but he was never charged.

'Banging the wall'

In his first broadcast interview since being released, Mr Aamer revealed details of the abuse he alleges was inflicted on him by American interrogators.

He also claimed a British intelligence officer witnessed him being beaten at a US air base in Afghanistan, where he was first held, but did nothing to stop the abuse.

Speaking to the Victoria Derbyshire programme, he said his interrogators at Bagram air base repeatedly banged his head against the wall.

"I felt my head - boom, boom, boom - banging the wall. And all I remember [is] that my head just kept banging the wall, back and forth, back and forth."

Asked if he was certain a British intelligence officer was present, he replied: "I would say 80, 90%. I have no doubt he is an Englishman, because of the way he spoke, the way he is very careful, the way he was sitting far away, looking at me."

He said the day before the incident he had "met John, who already told me 'I'm with the MI5 intelligence service and I came to ask you a few questions'. So, I have no doubt he was an Englishman."

Image copyright Family handout Image caption Shaker Aamer pictured with two of his children before he was detained

Soon after the 9/11 attacks on the US, Mr Aamer was detained in Afghanistan by bounty hunters tracking down and handing over possible al-Qaeda suspects. He was transferred to Guantanamo Bay in 2002.

Accusations against Mr Aamer were dropped in 2007 but it was another eight years before he was released. He has said he was in Taliban-controlled Afghanistan in 2001 to make a better life for his family.

Responding to the accusations against him, Mr Aamer said none was true: "You were an al-Qaeda operative? No, not at all.

"Prove it... prove anything that you say... prove it to the world," he said.

Reunited

Saudi-born Mr Aamer, who moved to the UK in the 1990s, told the Mail on Sunday he initially did not believe he was returning to England until he saw fields through the plane window as he was flown back in October.

He was reunited with his wife, Zinneera, that evening.

"At last, that moment I'd dreamt of came and she came through the door," he told the paper. "That instant washed away the pain of 14 years. It washed away the tiredness, the agony, the stress.

"It was like it no longer existed. I hugged her, she hugged me, and we just wept."

In his BBC interview, he described seeing his children again, including his 14-year-old son for the first time.

"They look at me and they're just trying to know, Who is this person? I feel like they are just looking at a stranger."

Analysis

Image copyright Reprieve UK

By BBC Home Affairs correspondent Dominic Casciani

Shaker Aamer's denunciation of modern-day jihadists is a hugely important moment in the fight for hearts and minds.

His generation came of age in the shadow of the Bosnia war where, in the mid-1990s, young men fought to protect European Muslims from certain genocide.

Mr Aamer says he did not fight in Bosnia, but those who did believed that their "jihad" was a legitimate and defensive "just war" - and in the UK none of them was ever accused of terrorism.

Others took a different path. They adopted Osama Bin Laden's interpretation of jihad - a war against the West which they blamed for virtually everything.

Extremists have long used the imprisonment of Mr Aamer - and others like him - as part of their recruitment drive.

His public demolition of their claimed justifications might just be the words that some on the cusp of violent extremism need to hear.

In the Mail interview, he claimed about 200 individual interrogators dealt with him in the time he was held.

He also claimed he had been tortured by being deprived of sleep and shackled to the floor in sub-zero temperatures.

Discussing terror attacks in the UK, he said: "How can you give yourself the right to be living here in this country, and living with the people and acting like you are a normal person, and then you just walk in the street and try to kill people?"

He added that killing civilians was not allowed according to his understanding of Islam.

"Even if there is a war you cannot kill just anybody, you cannot kill kids, you cannot kill chaplains, you cannot just go in the street and get a knife and start stabbing people," he said.

"If you are that angry about this country, you can get the hell out."

He also said he was concerned about a rift between Muslims and non-Muslims.

"It helps their [extremists'] cause," he said. "If you keep looking at people like they are terrorists before they do anything, then you will push them towards violence."

'Generosity of spirit'

Image caption Alex Salmond says Shaker Aamer has managed to come out of captivity with his sanity intact

Former SNP leader Alex Salmond told the BBC that Mr Aamer had emerged from "5,000 days of illegal captivity both sane and with a remarkable generosity of spirit".

Mr Salmond also said questions raised by Mr Aamer about what the UK government knew of his treatment by his captors needed to be addressed, saying: "I doubt if anyone reading that comprehensive interview... would doubt the veracity of his comments."

Watch the interview with Mr Salmond here.

A Foreign Office spokesman said: "The UK government stands firmly against torture and cruel, inhumane and degrading treatment or punishment.

"We do not participate in, solicit, encourage or condone it for any purpose. Neither does the UK make use of any so-called enhanced interrogation techniques. We have consistently made clear our absolute opposition to such behaviour and our determination to combat it wherever and whenever it occurs."

The US Department of Defense said it did not tolerate abuse of detainees, who were all treated humanely.

"All credible allegations of abuse are thoroughly investigated, and appropriate disciplinary action is taken when those allegations are substantiated," a spokesman said.

Mr Aamer has said he wants an apology from the US government over his treatment. He has also called for the UK government to hold an "open and transparent" inquiry into allegations that the UK was complicit in torture.

Mr Aamer is believed to be in line to receive compensation from the UK government, after deals were made with previous detainees.

Prime Minister David Cameron has already asked the government's Intelligence and Security Committee to investigate claims of UK complicity in rendition and torture at Guantanamo Bay.

Watch the Victoria Derbyshire programme's full interview with Shaker Aamer from 09:15 GMT on Monday, 14 December, on BBC Two and the BBC News Channel.