Teenage democracy activist Joshua Wong has been sent back to Hong Kong after being barred from entering Thailand, security officials said.
Officials said he was put on a Hong Kong Airlines flight back to the specially administered Chinese region about 12 hours after he arrived at Bankok's Suvarnabhumi Airport.
The 19-year-old activist rose to global prominence when he helped spearhead huge 2014 street protests against Beijing's plan to restrict elections in Hong Kong.
Mr Wong had been scheduled to give a talk at a university, but a Thai activist who had planned to greet him said he was informed by police that Mr Wong had been detained after Thai authorities received a request from the Chinese government.
Mr Wong, who turns 20 next week, was one of the high-profile student leaders behind pro-democracy protests two years ago that marked the former British colony's most turbulent period since China took control in 1997. In August, a Hong Kong court sentenced him to community service for his role in the protests, which brought parts of the city to a standstill for months.
Thai student activist Netiwit Chotipatpaisal said in a Facebook post that Mr Wong had "been confined at the immigration in Thailand because there's a request from Chinese government to Thai authority".
Thai Foreign Ministry spokesman Sek Wannamethee said permission for foreigners to enter Thailand "involves various factors and has to be in line with the relevant immigration laws and regulations". He added that the ministry is reviewing the facts with the Immigration Bureau and other authorities.
Nathan Law, who belongs to same political party as Mr Wong, Demosisto, urged Hong Kong justice secretary Rimsky Yuen to raise the issue with Thai officials on a previously scheduled three-day visit.
China's Foreign Ministry said in a brief statement that it was aware of reports of Mr Wong's detention, but did not say whether China had asked Thailand to detain him - only that it respected Thailand's ability to manage the entrance of people into the country "in accordance with law".
Refusing entry to Mr Wong would be in line with recent moves by Thailand's military rulers, who seized power in a 2014 coup.
The government has shown zero tolerance for dissent and has cracked down hard on its own student activists who have protested against the military rule.
It has detained students, stopped speeches from taking place and last month Thai authorities threatened to arrest Amnesty International speakers who were set to hold a news conference to release a report detailing allegations of torture at the hands of the military and police, causing the rights group to cancel the event.
Mr Wong was turned back in May last year when trying to enter Malaysia to speak at seminars in four cities. Malaysian officials said he was banned from entering the country but did not explain why.
Demosisto, which was founded earlier this year, advocates a referendum on "self-determination" on the future status of Hong Kong, which is in the middle of a 50-year transition period to Chinese rule.
Human rights activists called for Mr Wong's release.
"Thailand's arrest of Joshua Wong, a well-known pro-democracy activist in Hong Kong, sadly suggests that Bangkok is willing to do Beijing's bidding. Wong should be freed immediately and allowed to travel and exercise his right to free expression," said Sophie Richardson, China director at Human Rights Watch.
Two years ago, Mr Wong became one of the most prominent leaders of massive pro-democracy protests that shut down major thoroughfares in Hong Kong for 11 weeks. He and other youthful demonstrators demanded that the government drop a Beijing-backed plan to restrict elections for the city's top leader, but their movement fizzled out after authorities refused to grant concessions.
He was scheduled to give a talk at Chulalongkorn University about a new generation of political activism. Last month, Demosisto scored a stunning Hong Kong election victory when Mr Law, 23, won a seat in the legislature. Mr Wong was unable to join the race because he was too young under election rules.