A Hong Kong bookseller known for selling titles critical of Beijing told Thursday how he was blindfolded, kept in a cell and interrogated by Chinese authorities after going missing eight months ago.

Lam Wing-kee is one of five booksellers who published salacious titles about leading Chinese politicians and disappeared at the end of last year in a case that heightened fears Beijing was tightening its grip on Hong Kong.

He said that although he was not physically harmed, he had suffered mentally in detention and was unable to contact a lawyer or his family. He was kept in confinement, unable to walk outside and repeatedly interrogated.

The booksellers' case has sparked international condemnation.

Lam is the only bookseller to have spoken openly and said the case had "violated the rights of Hong Kong people".

He described how a confession he gave to Chinese state television about trading banned books was forced.

"I acted in front of the camera, I needed to. There was a director. I had to recite the script," he said.

"I was in fear. I felt helpless. I didn't know what they would do to me."

He was supposed to return to the mainland Thursday after being released to Hong Kong on bail Tuesday, he told reporters.

However, he decided not to go back and instead to speak out about his case after learning of the support the booksellers had received from the Hong Kong public.

Pro-democracy campaigners announced they would demonstrate against the detention of the booksellers on Friday morning outside mainland China's office in Hong Kong.

Lam said he had spent two sleepless nights making the decision to speak.

"If I don't speak up Hong Kong will be hopeless. It's not just a personal matter," he said.

"I dare not go back."

He added that while in custody he had been told that his case was being dealt with by a "special unit" which he believed was not part of the ordinary police or military.

- Fear of selling out -

The condition of his bail was to bring back a list of 600 customers, the majority of them from the mainland, who had received books from the Causeway Bay Bookstore that he managed.

Lam said he did not want to comply for fear that it would be seen as selling out the readers.

He struggled to contain his emotions as he gave a detailed description of his detention and how he was forced to sign a document giving up his right to a lawyer or to speak to his family.

Lam said he was arrested after crossing the border from Hong Kong into the southern mainland Chinese city of Shenzhen in October to see his girlfriend, but was not told why.

He was blindfolded and taken by train to the city of Ningbo, where he was kept in a 200-square-feet room for five months under guard. He was then moved to an apartment.

It was only some time after he was first detained that he was told his detention was related to bringing banned books into the mainland.

All but one of the booksellers have been allowed to return to Hong Kong on bail but have swiftly gone back over the border, apart from Lam.

The case of bookseller Lee Bo caused the greatest outcry as he was the only one to disappear from Hong Kong, leading to accusations that mainland law enforcement agents were operating illegally in the city.

Lee has insisted he is a free man just helping the authorities with their investigation.

But Lam said Lee had told him he had been brought to the mainland against his will.

The men all worked for the Mighty Current publishing house, which produced books about political intrigue and love affairs at the highest levels of Chinese politics.

Hong Kong was returned to China by Britain in 1997 and enjoys far greater liberties than in mainland China, but there are fears these are being eroded.