BEIJING (BLOOMBERG) - China on Wednesday (Aug 21) confirmed that it was holding a UK consulate employee in Hong Kong on allegations of violating local law, as protesters planned to rally for his release outside the diplomatic mission.

Chinese Foreign Ministry spokesman Geng Shuang said the consulate worker, Mr Simon Cheng, has been held under a 15-day administrative detention process in Shenzhen.

Mr Geng said the issue was an internal Chinese matter and not a diplomatic dispute, saying that Mr Cheng, 28, is a Hong Kong citizen.

Mr Cheng was reported missing after failing to return from an Aug 8 meeting in the adjacent city of Shenzhen.

The UK's foreign office said on Tuesday that it was "extremely concerned" and was seeking information from authorities in Hong Kong and the southern Chinese province of Guangdong, which includes Shenzhen.

In his remarks, Mr Geng cited China's Public Security Administration Punishment Law, a statute pertaining to minor violations. Individuals can be held under administrative detention for as long as 15 days, or roughly until Friday.

Mr Geng warned the UK against meddling in the affairs of its former colony. "The British side has made a lot of erroneous remarks on Hong Kong," he said, urging the UK "to stop pointing fingers and making accusations."

A "Save Simon Cheng" event is scheduled to take place on Wednesday evening at the UK Consulate General in central Hong Kong.

Mr Cheng's disappearance fuels concerns about the safety of diplomatic staff in China, already heightened by the December detention of Michael Kovrig, a Hong Kong-based security analyst on leave from Canada's foreign service. Mr Kovrig has since been accused of espionage and remains in secret detention, entitled to only monthly visits from Canadian diplomats.

The Chinese government has repeatedly said it respects international agreements protecting diplomats and that foreigners who abide by the country's laws have no reason to fear detention.

Mr Cheng is employed by the UK consulate and works for Scottish Development International, which encourages firms to do business with Scotland. He holds a British national overseas passport, the New York Times and others reported.

The British government returned Hong Kong to Chinese rule in 1997. Tensions between the two countries have simmered in recent weeks, after Beijing accused it of meddling in its former colony by defending the rights of anti-government protesters who have brought the city to a standstill since June.

The Hong Kong police said they launched a "missing person" investigation and were keeping "close contact" with Chinese authorities.