HONG KONG — Joshua Wong, who was a prominent leader of the 2014 pro-democracy rallies in Hong Kong, was detained at the international airport in Bangkok early Wednesday while trying to enter Thailand, he said after being put on a plane and flown back home.

In a brief news conference at Hong Kong International Airport, Mr. Wong, 19, said that more than 20 police and immigration officers met him on arrival in Bangkok, where he had been invited to speak at two universities. He said that his passport was confiscated and that he was detained with little explanation from the Thai authorities, who he said “mentioned a blacklist.”

“I’m disappointed in the Thai government,” he said. “I’m grateful now that I’m back in Hong Kong.” He said he felt lucky not to have met the fate of several Hong Kong booksellers who disappeared last year and later turned up in custody in mainland China.

News of Mr. Wong’s detention in Thailand had been reported Wednesday morning by a political party he recently helped establish, Demosisto. The party said he had arrived in Thailand at 11:45 p.m. on Tuesday on an Emirates flight.