HONG KONG • Hong Kong's leader has said he has asked China whether its handling of the booksellers case violated the "one country, two systems" formula under which the city returned to Chinese rule, the strongest response yet from the former British colony.

Chief Executive Leung Chun Ying told the Executive Council yesterday he had written a letter to Beijing asking whether the mainland authorities enforced their laws across the border in Hong Kong.

Thousands marched in Hong Kong last Saturday to protest against China's detention of five booksellers whose Hong Kong shop published gossipy books about Chinese leaders, including President Xi Jinping, in what critics called "cross-border abductions".

The arrests prompted fears that Beijing may be eroding the "one country, two systems" formula under which Hong Kong has been governed as a special administrative region since its return to China from British rule in 1997.

One of the booksellers, Mr Lam Wing Kee, said last week he had been held in captivity for eight months by Chinese agents. He claimed 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.

Mr Lam added that the 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.

Mr Leung told reporters his letter asked Beijing to clarify how the 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.

"Did the handling of the incident hinder the 'one country, two systems' principle and the Basic Law protecting Hong Kong residents' freedom and rights, especially the freedom of expression, publication and personal safety?" he said.

Hong Kong's freedoms are protected by the Basic Law, a mini-Constitution that includes the "inviolable" freedom of Hong Kong people from arbitrary arrest and search.

Beijing's Liaison Office in Hong Kong did not immediately respond to a request for comment. Chinese Foreign Ministry spokesman Hua Chunying, when asked about the case, said China abided by the "one country, two systems" policy and that Hong Kong residents enjoyed full rights and freedoms.

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

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

Mr Gui remains in custody in China, while Mr Lee has insisted he is a free man voluntarily helping the investigation.

Two other booksellers briefly returned to Hong Kong on bail, but then travelled back to China.

REUTERS, AGENCE FRANCE-PRESSE