It said that he was questioned and was not allowed to use his cellphone or computer. It said he was expected to be sent back to Hong Kong on an Emirates Airline flight later on Wednesday as that was the airline he flew to Thailand with.

An immigration official confirmed to AFP that Mr Wong had been detained but declined to say who issued the order to deny him entry.

Thai junta spokesman Colonel Winthai Suvaree said he was not aware of any order from Beijing to detain the activist.

China's Foreign Ministry, when asked about Mr Wong's detention, said it had "noticed the relevant reports," news agency Reuters reported.

"China respects Thailand's exercise of immigration control according to the law," it said in an email.

On Tuesday the nineteen-year-old had expressed concern about his trip to Bangkok in a Facebook post.

"We all know Thailand is not politically stable... It is also clear that it is close to the Chinese Communist Party," he said.