Hong Kong's Chief Executive Leung Chun-ying said his government will write a letter expressing concern over booksellers' case to China's central authorities.

Leung told journalists during a news conference that "only Hong Kong's law enforcement agencies have the authority to enforce the law." 

"I chaired a meeting this morning and made three decisions. One is to write to the central authorities to express our concern over this case," Leung added. 

One of five Hong Kong booksellers who went missing in mysterious circumstances told media on Thursday (June 16) he had been detained for more than eight months by Chinese authorities and that another of the five had been abducted from Hong Kong.

Over 4,000 people protested against their detention on Saturday (June 18).

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.

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