Hong Kong democracy activist Joshua Wong held in Thailand

The bespectacled Wong, 19, famed for his galvanising role in the city's 2014 pro-democracy "umbrella movement", was immediately held upon arrival at Bangkok's Suvarnabhumi airport late Tuesday, according to Thai student activist Netiwit Chotipatpaisal. An immigration official confirmed the detention.

"I have been told Joshua will be sent back to Hong Kong," Netiwit, who had been due to meet the pro-democracy champion, told AFP from the airport Wednesday morning.

According to Netiwit's Facebook page, police said that "there was a written letter from the Chinese government to the Thai government concerning this person".

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

"It's not yet 100 percent whether he will be deported this afternoon. We are waiting for airline confirmation but normally the airline which brought him here will be responsible," the official said, requesting anonymity.

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

Wong's party, Demosisto, said in a statement it could not contact Wong for more than nine hours and "strongly condemns the Thai government for unreasonably limiting Wong's freedom and right to entry, and requests the immediate release of Wong".

"We are deeply worried about Wong's personal safety and freedom, and we remain uncertain as to under what circumstances Wong has been detained," it added.

Wong's Demosisto colleague, 23-year-old Nathan Law, who last month became Hong Kong's youngest lawmaker, told AFP: "It is quite shocking that we still cannot connect with him for this long period of time."

'Serious political pressure'

Law said he had concerns that Wong could be sent to mainland China, as happened with a book seller who became a thorn in the side of Beijing and was detained in Thailand last year while on holiday.

However, he said it was more likely that he would be sent back to Hong Kong.

"He's a very high profile politician and if the Chinese government did so, then they would have to bear serious political pressure," Law said.

Last year Wong was similarly barred from entering Malaysia, where officials sent him back to Hong Kong citing fears his planned talks would damage ties with Beijing.

Thailand's military leaders have cosied up to regional superpower China since their 2014 power grab, with Beijing offering diplomatic support and foreign investment.

Wong's detention comes almost a year after Hong Kong bookseller Gui Minhai disappeared in Thailand.

He later resurfaced in China, one of a number of men detained by Chinese authorities over their involvement in the selling of titles critical of Beijing's leaders.

Wong was among three student leaders convicted in August over the 2014 storming of the forecourt of Hong Kong government headquarters, an event that preceded mass protests that brought much of the city to a standstill for more than two months.

This year, he co-founded Demosisto, a party calling for a referendum on Hong Kong's future, including the option of independence -- angering China, which holds a tight grip over the semi-autonomous city.

Wong had been due to speak at an event in Bangkok on Thursday marking the 40th anniversary of a massacre of pro-democracy students by security forces and royalist militias.

The Thammasat massacre of October 6, 1976, remains a deeply sensitive issue in Thailand's blood-flecked recent history.