One of five Hong Kong booksellers who went missing in mysterious circumstances last year has said he had been held for more than eight months by Chinese authorities.

Lam Wing-kee announced on Thursday that he was arrested in the southern Chinese city of Shenzhen and that his colleague, Lee Bo, who went missing from Hong Kong in December, had also been abducted.

Following months of speculation about the circumstances surrounding the disappearances, Lam called a surprise press conference just two days after being released.

Lam said he was taken on a 14-hour train journey to the eastern city of Ningbo following his arrest.

There, he was kept in a small room by himself, and repeatedly interrogated about the selling of banned books on the mainland.

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Causeway Bay Books, the store at which the five men worked, had specialised in publishing and selling gossipy books about China's leaders, including President Xi Jinping.

"At the time I was terrified ... I didn't know how they'd treat me," he said in Hong Kong. "I couldn't believe this was happening.

"If I myself, being the least vulnerable among the five booksellers, remained silent, Hong Kong would become hopeless.

"It took me much courage and two sleepless nights to consider, but I decided to share the whole story with you, and tell the whole world that this incident is not only about myself or the bookstore, it's about the core values that Hong Kong people need to safeguard."

Suspicions about China

The disappearances have prompted fears that mainland Chinese authorities may be using tactics that erode the "one country, two systems" formula under which Hong Kong has been governed since its return to China from British rule in 1997.

Al Jazeera Florence Looi, reporting from Beijing, said that despite China saying none of its officials were involved in the abductions, there were always suspicions that Chinese officials were involved.

"What Lam has said has once again heightened fears of China's interference in Hong Kong's affairs despite promising it a high degree of autonomy when it took back the former British colony in 1997," she said.

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"Two of the five abducted are foreign passport holders, and there are fears that Chiina is overreaching its legal rights."

Four of the men - Gui Minhai, Lui Por, Cheung Chi-ping and Lam - gave details of their alleged offences to China's Phoenix Television in February, saying they had been detained for "illegal book trading" in mainland China.

But Lam said this interview had been "scripted" by Chinese agents and that he had been forced to say what they demanded.

'Concerted operation'

Amnesty International, the UK-based rights group, said in a statement that Lam had "exposed what many have suspected all along: that this was a concerted operation by the Chinese authorities to go after the booksellers".

Lam said he was barred from calling his family or contacting a lawyer during his detention, while being monitored 24 hours a day.

He was later transferred to Shaoguan, a city in the southern Chinese province of Guangdong.

The Hong Kong government said in a statement that the police were reaching out to Lam and would take appropriate action.

Chinese authorities have declined to clarify key details of the disappearances but have said previously that law enforcement officials would never do anything illegal.

Wang Chaoye, an official of China's main representative office in Hong Kong, declined to comment on Lam's testimony when contacted by Reuters news agency.

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Source: Al Jazeera and agencies