An activist and a Thai immigration official said Bangkok responded to a request from Beijing, though a Thai government spokesman has denied that.
Mr Wong, a 19-year-old activist who rose to global prominence by spearheading the 2014 street protests against Beijing's plan to restrict elections, arrived around midnight Tuesday at Bangkok's main airport. He said he was immediately taken into custody by 20 police and immigration officers and then put on a flight back to Hong Kong about 12 hours later.
He was due to give a talk at Chulalongkorn University about lessons from Hong Kong's “Umbrella Movement” protests as part of Oct. 6 commemorations of a Thai government crackdown on student demonstrators 40 years ago.
Netiwit Chotipatpaisal, a Thai activist who had planned to greet him at the airport, said police informed him that Wong was detained after Thailand received a notice from the Chinese government.
The deputy commander of Suvarnabhumi Airport's immigration office, Pruthipong Prayoonsiri, said Wong was blacklisted after China asked the Thai government to deny him entry, according to a report in The Nation newspaper. An officer at the airport department, speaking on condition of anonymity because staff are prohibited from commenting to media, confirmed that Pruthipong made the comments at a briefing.
Prime Minister Prayuth Chan-ocha told reporters that “it's China's business,” without elaborating on how Thai officials made the decision to deny Mr Wong entry.
Government spokesman Sansern Kaewkamnerd, however, said there was no order to detain Mr Wong. He said authorities were aware 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.”
Mr Wong, who turns 20 next week, was one of the 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.
Mr Wong said authorities took his passport and “illegally detained” him in a windowless holding cell at the Bangkok airport. He said he was not given a clear explanation for his detention and was not allowed to contact his family or a lawyer.
“I actually had a lot of discussion with a Thai official, but because he didn't speak English very well, I couldn't hear him very well. But there was one word I heard very clearly: blacklist,” he told reporters after arriving at Hong Kong's airport.
Mr Wong, who last year was prevented from entering Malaysia, said he was relieved he did not end up like five Hong Kong booksellers who disappeared and later turned up in custody in mainland China. While four have been freed, Chinese-born Gui Minhai, a naturalised Swedish citizen who vanished from his holiday home in Thailand, is still detained.
“If I hadn't returned to Hong Kong, I can't imagine what kind of situation I'd be in,” Mr Wong said. “Fortunately, I did not become another missing person.”
At a news conference later, Mr Wong said: “Of course I think Beijing played an important role in this incident,” which he believed was a result of closer ties between Thailand and China.
The bookseller case and other incidents have intensified fears in Hong Kong that Beijing is overstepping its boundaries and undermining a “one-country, two systems” formula that governs the territory's relationship with the mainland.
Nathan Law, who co-founded the political party Demosisto with Wong earlier this year, demanded an explanation and apology from the Thai government. He condemned Beijing for “exerting inappropriate influence and diplomatic pressure” on Thailand.
Mr Law, 23, was elected Hong Kong's youngest legislator last month while Mr Wong, who is Demosisto's secretary general, was two years too young to run under election rules. Their party wants 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.
Amnesty International said the decision to block Mr Wong “underscores the government's willingness to suppress the right to freedom of expression and raises serious concerns about how China is using its influence over Thai authorities.”
China's Foreign Ministry said in a brief statement that it was aware of the case, 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.”
Mr Wong's detention “sadly suggests that Bangkok is willing to do Beijing's bidding,” said Sophie Richardson, China director at Human Rights Watch.
Refusing entry to Mr Wong would also 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 the military rule. It has detained students and stopped speeches from taking place. Last month, Thai authorities threatened to arrest Amnesty International speakers who planned 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.
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