China has said an employee at the British consulate in Hong Kong has been given 15 days of administrative detention in the city of Shenzhen for violating regulations on public order.

The case has stoked fears among Hong Kong residents that Beijing is extending its judicial reach to Hong Kong, a semi-autonomous Chinese territory and former British colony.

“The relevant employee is a Hong Kong resident, not a British citizen,” Chinese Foreign Ministry spokesman Geng Shuang said at a daily briefing. “In other words, he is Chinese. This is purely the internal affairs of China.”

Mr Geng said the man violated regulations on “Punishments in Public Order and Security Administration”, but gave no further details, adding that his case is “not a diplomatic issue”.

About two dozen people gathered on Wednesday outside the British consulate in Hong Kong to demand the UK Government step up efforts to secure the release of Simon Cheng Man-kit, a trade and investment officer at the consulate.

A supporter holds a poster outside the British Consulate in Hong Kong
A supporter holds a poster outside the British Consulate in Hong Kong (Vincent Yu/AP)

“Save Simon now!” the crowd chanted in front of a wall of missing person posters with pictures of Cheng.

Max Chung, the event’s organiser and an acquaintance of Mr Cheng, said he had travelled to the neighbouring city of Shenzhen on a mainland travel permit for Hong Kong and Macao residents.

He holds a British National Overseas passport, according to Mr Chung. The travel document is issued by the UK to Hong Kong permanent residents.

Public order and security infractions are generally minor — for instance, fighting in the street — and result in administrative rather than criminal punishments. The maximum penalty is 15 days in detention.

The British Foreign Office has said it is “extremely concerned” about its staff member. Local media said he attended a business event in Shenzhen on August 8 but never returned to Hong Kong despite plans to do so the same day.

Pro-democracy protesters
Pro-democracy protesters (Kin Cheung/AP)

The seizure of consular staff of any status or rank is highly unusual. Despite Beijing’s declaration of a “golden era” in Sino-British ties, relations have grown tense in recent months amid pro-democracy protests in Hong Kong, a former British colony.

“Regarding the series of words and deeds by the British side recently on Hong Kong-related issues, we have repeatedly lodged solemn complaints with the UK,” Mr Geng said, adding that China urges the UK to “stop meddling in Hong Kong affairs”.

Mr Chung and other friends of Mr Cheng said they believe he was detained at the high-speed railway station in West Kowloon, part of Hong Kong. The station stirred controversy ahead of its opening last September because passengers go through Chinese immigration and customs inside it.

Mainland law applies in the customs area, sparking concerns among Hong Kongers over the ability of mainland authorities to enforce Chinese law on Hong Kong soil. Some said this is a breach of the “one country, two systems” framework, which allows Hong Kong to largely run its own affairs until 2047.