Hong Kong pro-democracy activist Joshua Wong has been detained by the Thai authorities in Bangkok, according to the political party he heads.
Mr Wong was detained at Suvarnabhumi airport after landing there on a flight from Hong Kong, Demosisto said in a statement.
It cited Thai student activist Netiwit Chotipatpaisal, who had invited him to Bangkok and had been waiting for him.
Joshua Wong became the face of Hong Kong's pro-democracy protests in 2014.
He was at the heart of the mass demonstrations, which came to be known as the Umbrella Movement, that brought parts of the city to a standstill for nearly three months.
Demosisto, the party he later founded, calls for Hong Kong's right to self-determination.
The 19-year-old activist had been invited by Mr Netiwit to address students at Chulalongkorn University to mark the 40th anniversary of a massacre of students in 1976.
Mr Netiwit said that Thai authorities had "received a letter from the Chinese government" regarding Mr Wong's visit, the Demosisto statement says.
When speaking about Mr Wong's impending visit, Mr Netiwit earlier told the South China Morning Post newspaper that Thailand could hold protests similar to the Umbrella movement.
Thailand remains under a military government since a 2014 coup.
Last May Mr Wong was denied entry to Malaysia where he had been set to take part in talks about democracy in China. He was immediately returned to Hong Kong.
Observers say it is likely he will be sent back to Hong Kong in this instance too.
In recent years Thailand has faced criticism for its co-operation with the Chinese authorities.
Last year the UN condemned the Thai authorities for deporting two Chinese dissidents who had been given refugee status by the organisation. The two men had been in detention in Thailand for illegally entering the country and rights group believe they were returned to China.
Thailand also sparked international consternation when it sent back dozens of Uighur Muslims who had fled China. The government in Bangkok insisted it acted within international law.