China confirmed that it was holding a U.K. 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 Wednesday that the consulate worker, Simon Cheng, was being held under a 15-day administrative detention process in the mainland city of Shenzhen. Geng said the issue was a domestic matter and not a diplomatic dispute, saying that Cheng, 28, is a Hong Kong citizen.

Cheng was revealed to be missing Tuesday after failing to return from an Aug. 8 meeting in Shenzhen and hasn't contacted his family since. The U.K.'s foreign office said 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.