HONG KONG (Reuters) - Hong Kong’s leader said on Tuesday he had 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.

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

Thousands marched in Hong Kong on 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 Beijing may be eroding the “one country, two systems” system 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, Lam Wing-kee, said this week he had been held in captivity for eight months by Chinese agents. [L4N19C1AG]

Leung said he sought assurance in the letter that if Hong Kong residents are detained on the mainland, their legal rights are protected and questioned whether an existing Hong Kong-mainland notification system was transparent enough.

“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 authorities have repeatedly said they would never do anything illegal and that Hong Kong’s autonomy is fully respected.

Chinese Foreign Ministry spokeswoman 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.

A number of Western governments, including Britain, voiced concerns this year that Chinese-born British national Lee Bo, who went missing from Hong Kong in late December, had been abducted.

Hong Kong protesters on Saturday chanted “protect freedom of the press, freedom of publishing and freedom of speech” as they marched from the Causeway Bay Books shop to the Liaison Office.

They also demanded the release of Gui Minhai, a Swedish passport holder who disappeared from the Thai resort of Pattaya last October and is the only one of the five still in detention in China.