Thailand denied permission for Hong Kong democracy campaigner Joshua Wong to enter the country, reportedly at China's behest.
Wong, 19, has been a thorn on China's side ever since he emerged as the face of the pro-democracy protests in Hong Kong in 2014.
The student leader, who had been convicted of charges relating to the "umbrella movement" that paralysed the city two years ago, was scheduled to attend an event in Bangkok commemorating the massacre of student activists.
Channel News Asia reported that Wong was detained at Bangkok's Suvarnabhumi Airport on Tuesday night.
The news of the activist's detention immediately set off speculation that Thai military junta was acting under China's orders, AFP reported.
The agency cited a Thai student organiser saying that police told him about a "written letter from the Chinese government to the Thai government concerning this person".
"I have been told Joshua will be sent back to Hong Kong," Thai student activist Netiwit Chotipatpaisal added.
However, the government officials said they did not know about any instruction from Beijing regarding Wong's travel to Thailand.
In July this year, the student leader was convicted for leading mass pro-democracy protests. He was found guilty of illegitimate assembly and climbing into the Civic Square, a Hong Kong government complex forecourt, on September 26, 2014, along with other students.
However, an appellate court in the former British colony stopped short of handing out a prison term to Wong and the fellow leaders.
Disappearance of a Hong Kong book seller
Demosisto, the political party Wong founded, condemned the detention of Wong. In a Facebook post, Demosisto said it "strongly condemns the Thai government's intervention in limiting Wong's freedom and right to entry".
It also called for the immediate intervention of Hong Kong authorities to ensure Wong's release and safety.
The concerns of Wong's followers and pro-democracy activists in Hong Kong were heightened in the backdrop of the disappearance of a Hong Kong book seller in Thailand last year.
The book seller, who had published material critical of Chinese leaders, later resurfaced in China, hinting at Beijing's apparent role in his disappearance.
Thailand had deported more than 100 Uighurs fleeing Chinese oppression last year.