Defying China, Lam Wing-kee, who resurfaced earlier this week, spoke publicly about his detention by Chinese authorities at a surprise news conference, according to Hong Kong public broadcaster RTHK.
Lam said he was taken by "special forces" after crossing the border into mainland China from Hong Kong eight months ago and detained in a small room. A confession he made on Chinese state television was scripted and edited, he added.
He said he had been told to return to mainland China on Friday with evidence about to whom his bookstore had been sending banned books. But he said he had decided not to go back and wanted to speak out about what had happened.
"I had to be very courageous. I thought about it for two nights before I decided tell you all what happened, as originally and completely as I could," he said, according to a transcript translated by the South China Morning Post.
"I also want to tell the whole world. This isn't about me, this isn't about a bookstore, this is about everyone."
Lam said, unlike the other booksellers, he had no family in China and this made it easier to speak up.
"I would refer you to competent authorities handling this case. He is a Chinese citizen who violated law in the mainland and we have the right to deal with the case in accordance with law."
The Hong Kong government said in a statement that police would contact Lam to "understand more" and said the government "attaches importance to the personal safety of every Hong Kong resident."
Daughter speaks out
All five of the booksellers were involved with publisher Mighty Current and its shop Causeway Bay Books, which sold gossipy titles about China's elite.
The disappearance of the booksellers sparked outrage in Hong Kong and internationally over fears they were taken against their will by Beijing authorities.
Lam and three other booksellers were allowed to return to the Hong Kong, but Gui Minhai -- who holds a Swedish passport -- remains missing.
Gui's daughter, Angela Gui, told CNN Thursday, in her first broadcast interview, that she has received some messages and phone calls from her father in which he said she should keep quiet about his plight.
Story 'blown apart'
Rights group Amnesty International said that China must explain what happened to the five men.
"Lam Wing-kee has blown apart the Chinese authorities' story. He has exposed what many have suspected all along: that this was a concerted operation by the Chinese authorities to go after the booksellers, " said Mabel Au, Director of Amnesty International Hong Kong.
"It seems clear he, and most likely the others, were arbitrarily detained, ill-treated and forced to confess."
Lam said he was handcuffed and blindfolded as soon as he crossed into China from Hong Kong, a former British colony which is now an autonomous region of China with a separate legal system.
The bookseller said that at first no one explained why he was being detained. Later, Lam said he was told that his acts of buying and selling books banned in mainland China were illegal and that he faced prosecution.
CNN's Kristie Lu Stout in Hong Kong and Tim Schwarz in Beijing contributed to this report