A former opposition candidate in the Belarus presidential election has alleged that he suffered human rights abuses while in detention.
Ales Mikhalevich, who was arrested after a crackdown on the opposition in December, accused security forces of using both physical and mental torture.
Mr Mikhalevich, 35, said he was put on a stretch vice in handcuffs for refusing to condemn opposition actions.
He was released a fortnight ago and faces charges of inciting mass riots.
Mr Mikhalevich is one of five former candidates and 37 opposition activists to be accused of incitement after tens of thousands of people took part in protests after the 19 December vote described by international monitors as "lacking transparency".
Thirty-three of the accused remain in detention. Some have not even been seen since they were arrested in December.
In a written statement, Mr Mikhalevich gives a vivid description of how he was subjected to what he calls "sleep-deprivation torture". Prisoners were allegedly forced to sleep under lamps and forbidden to cover their faces, he said.
In an interview with BBC News, the former presidential candidate said he still found it difficult to walk and had problems with his eyesight after the alleged physical abuse.
"I had to go public with what happened to me," he said. "This is not about me, not about my political future. This is about my duties as a citizen and as a human being."
Despite the threat of re-arrest, Mr Mikhalevich said he could not bear to look at himself in the mirror every day knowing what was happening to some of his colleagues inside Belarusian prisons.
At a press conference in Minsk, Mr Mikhalevich explained how security officials said they would set him free if he agreed to become a KGB agent and inform on the activities of the opposition.
"I agreed immediately," he said. "Not because I had any intention of actually becoming an informant. But because I need to tell the world what is happening in Belarus."
He said he was aware he could end up back in the KGB detention centre facing far harsher treatment.
A spokesman for the KGB denied Mr Mikhalevich's accusations and told Belarusian journalists the allegations of torture were untrue.
Natalia Koliada is one of those who were detained on the night of the election. She is now free and has fled Belarus. She said she was in detention for 14 hours and not allowed water, food or sleep.
Ms Koliada explained how the guards shouted they wanted to rape and kill her, and how those detained were not held in a cell but had to stand in freezing prison corridors.
She said they were not allowed to use the toilet. Instead they were forced to defecate in front of members of the opposite sex, she said.
These allegations of torture are now central to an international law suit being mounted against Belarusian President Aleksandr Lukashenko and his government.
Lawyers in London are collating evidence from those who were allegedly beaten during the protest and detained, and have written letters of intent which have been presented to various United Nations bodies, including the UN Committee Against Torture.
It is just one in a series of moves being planned by the newly-formed Free Belarus Now campaign group. A number of high profile names have signed up to it, including former Soviet President Mikhail Gorbachev, and Hollywood stars such as George Clooney and Sigourney Weaver.
Elena Edwards is a founding member of the group and sister-in-law of Ales Mikhalevich. She told BBC News she was extremely proud of him, if also very worried.
"This courageous move came at the right moment," she said. "Just as everybody had started to forget about Belarus."