Thailand deported Hong Kong student activist Joshua Wong on Wednesday over "national security" concerns, the Thai prime minister's office said in a statement.
Wong, the secretary general of the Hong Kong pro-democracy political party Demosisto, was detained late Tuesday upon his arrival at Bangkok's Suvarnabhumi International Airport.
"Mr Wong had been active in resistance movements against other foreign governments, and ... if such actions were taken within Thailand, they could eventually affect Thailand's relations with other governments," the statement read.
The activist had been due to speak at a youth political forum at a Bangkok university coinciding with the 40th anniversary of a massacre of Thai student protesters on October 6, 1976.
Wong played a key role in the so-called "Umbrella Movement" protests of 2014 that saw parts of Hong Kong occupied for 79 days. The demonstrations posed the biggest challenge to China's communist government since Hong Kong was returned from Britain in 1997.
Wednesday's statement from the Thai government stated officials were "aware of this incident [Wong's deportation] but has not maintained any policy or specific directive on this matter."
The statement was issued by Lt Gen Sansern Kaewkamnerd. Thailand has been ruled by a military government since a May 2014 coup.
A Thai student activist said earlier that police at the Bangkok airport told him the detention was due to a Chinese government request.
Thai student activists with the New Democracy Movement protested Wong's deportation outside the Chinese embassy Wednesday afternoon.
"This is not the first time the Thai government has disregarded human rights for the interests they would gain from the Chinese government," one of students told reporters, citing a July 2015 incident in which Thailand deported more than 100 ethnic Uighur asylum-seekers.
The Thai government statement did not address possible involvement of the Chinese government.
In an emailed response to dpa, a Chinese Foreign Ministry spokesperson said, "The Chinese side respects Thailand's domestic implementation of entry and exit management based on the law."
Thai student activist Netiwit Chotiphatphaisal had invited Wong to speak in Bangkok because he hoped that Thais could follow his example as a young political leader.
"Thai youths see politics as something intimidating that shouldn't be discussed," he said. "We want our fellow students to learn and change that mindset. The Thai government should have given Thai students a chance to learn from what Joshua Wong has to say."
Rights groups criticized the Thai government's refusal to allow Wong into the country.
"Thailand's arrest of Joshua Wong ... sadly suggests that Bangkok is willing to do Beijing's bidding," Sophie Richardson, China director for Human Rights Watch, said earlier.
"Wong should be freed immediately and allowed to travel and exercise his right to free expression," Richardson said.
Wong's detention and deportation comes at a time when Hong Kong's citizens are increasingly concerned that China is cracking down on critics in the former British colony, and that foreign countries are unwilling to challenge Beijing's actions.