Hong Kong student leader Joshua Wong was barred from entering Thailand early yesterday at China's request, said Thai Prime Minister Prayut Chan-o-cha.
Mr Wong was detained for about 10 hours after arriving at Bangkok's Suvarnabhumi airport. He took a flight back to Hong Kong yesterday afternoon.
At a press conference last night, the 19-year-old protested against the "illegal detention" and said that Thai officers at the airport had confiscated his passport and told him he was blacklisted.
"I asked them what offence have I committed, but they did not answer me and refused my requests to contact a lawyer or call my parents."
When he pressed for a reason, he said he was told: "This is Thailand, same as China, not Hong Kong."
Mr Wong, a leader of the so-called Umbrella Movement in 2014 and co-founder of Hong Kong political party Demosisto, had been invited to give a talk at Chulalongkorn University, to commemorate the 40th anniversary of a right-wing massacre of leftist students in Thammasat University in 1976.
In Bangkok, Mr Prayut was quoted as saying that Mr Wong's expulsion was "China's issue".
"Officials there have requested to take him back. It's Chinese officials' business," he was quoted as saying by Bloomberg News.
Earlier, a government spokesman said the Thai authorities decided to deny Mr Wong entry into the country for fear of hurting its "relations with other nations".
"Mr Wong had been active in resistance movements against other foreign governments, and that if such actions were taken within Thailand, they could eventually affect Thailand's relations with other nations," said the spokesman.
Recounting his detention in a 50 sq ft cell at a police station in the airport, Mr Wong said: "I was worried if I would become the next Gui Minhai or face a similar fate as the Causeway Bay booksellers."
Mr Gui and four other men, who ran a publishing firm and bookshop in Causeway Bay specialising in salacious gossip about China's leaders, went missing last year.
Mr Gui, a Swedish citizen, was in Thailand when he vanished. Early this year, he said in a TV confession that he had surrendered to the Chinese authorities to face action for a fatal drink-driving accident.
Political analyst Willy Lam said Mr Wong's case is "a very disturbing sign".
"For China to pull strings just to target a... college student who is not a criminal but who has a different vision for Hong Kong from Beijing is seen as a heavy-handed and disproportionate use of power," Dr Lam told The Straits Times, adding that the incident will exacerbate the tension between Hong Kong and China.
Mr Wong said he will now think twice about visiting countries that have extradition agreements with China and does not see himself stepping foot into Malaysia or Thailand again.
Though he will not be in Bangkok, he said he will try to share his experience with Thai students via Skype.