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."