Hong Kong's leader said Tuesday he has raised concerns with China about the case of a bookseller who reported being detained for eight months on the mainland, amid fears Beijing is tightening its grip on the city.

Lam Wing-kee was one of five employees of a Hong Kong firm -- which published gossipy books about leading Chinese politicians -- to go mysteriously missing last year. All later emerged on the mainland.

Lam has said he was seized just across the border from Hong Kong, taken away blindfolded and then kept in a cell, under interrogation and without access to his family or a lawyer, for alleged involvement in bringing banned books into the mainland.

Hong Kong's Beijing-backed Chief Executive Leung Chun-ying, who has been accused of dragging his feet over the issue, said he had written to Beijing relaying local concerns.

A leading pro-democracy activist called the government "pathetic", and said its actions failed to allay residents' fears that the semi-autonomous city's freedoms are being eroded.

Lam, who was placed on suicide watch during his detention, made his explosive disclosures about his detention in Hong Kong last Thursday.

He said Chinese authorities had allowed him to return home to collect a list of mainland customers for the banned books, but he is refusing to go back across the border.

Leung told reporters his letter asked Beijing to clarify how relevant mainland departments handle cases in which Hong Kong people have broken mainland laws, and whether or not mainland personnel had carried out cross-border law enforcement in the city.

Mainland law enforcers are barred from operating in the city under the "One country, two systems" agreement governing Hong Kong's return from Britain to China in 1997.

Leung said his letter also asked "if the handling of the issue affected One country, two systems and the Basic Law, which guarantees the freedoms and rights of Hong Kong people".

When asked about Leung's remarks, a Chinese foreign ministry spokeswoman said China will stick to the handover arrangement.

"I would just like to reiterate the Chinese government's firm determination to uphold the One country, two systems (principles) is steadfast. The people of Hong Kong enjoy their full rights and freedoms in accordance with the law," Hua Chunying told reporters.

- 'Expected' response -

Civic Party lawmaker Claudia Mo described the Hong Kong government's response as "pathetic, as everyone's expected it to be".

"Leung is quite terrified obviously, to take up the issue clearly and loudly with the Beijing authorities; he obviously wants to make sure that he doesn't embarrass his master in any way," Mo told AFP.

"This is probably the most striking, the most unsettling case that actually hampers the One country, two systems promise."

Lam, 61, likened his ordeal to Cultural Revolution-era repression during an interview with AFP Sunday.

He told a local radio station he had no hopes Leung's follow-up actions would bring about change.

"The Hong Kong and the Chinese governments are not on equal terms," he said.

Lam and two other booksellers went missing in the city of Shenzhen just across the border from Hong Kong, while a fourth -- Gui Minhai -- disappeared in Thailand and a fifth -- Lee Bo -- went missing in Hong Kong itself.

Three of the other booksellers have disputed parts of Lam's account. Lee Bo has denied he told Lam that he (Lee) was taken to the mainland against his wishes.

Gui remains in custody in China while Lee has insisted he is a free man voluntarily helping the investigation into the smuggled books.

Two other booksellers who were detained in China have briefly returned to Hong Kong on bail, but then travelled back to the mainland.