HONG KONG — A Chinese-born billionaire who has forged financial ties with some of the country’s most powerful families was taken by the Chinese police from his apartment at the Four Seasons Hotel in Hong Kong late last week and spirited across the border, a person close to the businessman said on Tuesday.

The billionaire, Xiao Jianhua, who has been missing since Friday, is in police custody in China, where he apparently is safe, said the person, who spoke on condition of anonymity for fear of arrest. Mr. Xiao is a Canadian citizen with an Antiguan diplomatic passport, though he was born in China.

His removal from Hong Kong appears to contravene the “one country, two systems” rule that allows the former British colony to run its own affairs and bars the Chinese police from operating here.

When asked on Tuesday about Mr. Xiao, the Hong Kong police issued a statement saying that “the subject” had entered mainland China through a border crossing on Friday. His disappearance was reported the following day, but on Sunday one of Mr. Xiao’s family members reported that Mr. Xiao was safe and that the family had asked to withdraw the missing person’s filing.