By Stella Tsang and Clare Baldwin

HONG KONG (Reuters) - One of five Hong Kong booksellers who went missing in mysterious circumstances last year said on Thursday he had been detained for more than eight months by Chinese authorities and that another of the five had been abducted from Hong Kong.

Lam Wing-kee told a news conference that his colleague, Lee Bo, who went missing from Hong Kong in late December, had been abducted, and said "cross-border enforcement actions" by mainland Chinese authorities in Hong Kong were "not acceptable".

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 arrested last October in the southern Chinese city of Shenzhen and then taken on a 14-hour train journey to the eastern city of Ningbo. There, he was kept in a small room by himself, and repeatedly interrogated about the selling of banned books on the mainland.

Causeway Bay Books, the store at which the five men worked, had specialized 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 told a roomful of reporters in Hong Kong. "I couldn't believe this was happening."

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.

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'd been detained for "illegal book trading" in mainland China.

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

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 with China's main representative office in Hong Kong, declined to comment on Lam's testimony when contacted by Reuters.

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 bespectacled Lam, who appeared tired after deliberating for two days on whether to go public, said the case had "clearly violated Hong Kong's rights", and that he needed to speak out no matter what the risks to his personal safety or that of his mainland Chinese girlfriend.

"I hope the central government will not ... do something against my friends."

Lam said he was asked by the Chinese officers who detained him to return to Hong Kong to retrieve records of customers who'd ordered banned books from his bookshop. But Lam said he would never return to China.

Pro-democracy lawmaker Albert Ho urged the Hong Kong government to follow up on the case with Beijing, saying it remained of great concern internationally. Lee Bo is a British national and went missing in Hong Kong, while Gui Minhai is a Swedish citizen and went missing while in Thailand.

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

Amnesty International 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."

Only Gui Minhai remains in Chinese custody, though the precise nature of his charges have not yet been clarified.

(Reporting by Clare Baldwin, James Pomfret and Stella Tsang; Writing by James Pomfret; Editing by Catherine Evans)