Thailand stopped teen pro-democracy activist Joshua Wong from entering the country and sent him back to Hong Kong, officials said Wednesday, in a move supporters suspected was triggered by pressure from Beijing.

Thai officials and Wong's political party said he was put on a Hong Kong Airlines flight back to the specially administered Chinese region on Wednesday about 12 hours after he arrived at Bangkok'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.

Wong, who arrived late Tuesday, 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.

But Netiwit Chotipatpaisal, a Thai activist who had planned to greet him, said he was informed by police that authorities detained Wong after receiving a notice from the Chinese government.

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.

In response to questions about Wong, Thai Foreign Ministry spokesperson 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."

If it's true Beijing leaned on Thailand, "it will seriously damage the reputation of the Chinese government and it will show a very bad example of how the Chinese government deals with human rights defenders in Hong Kong," said Nathan Law, who co-founded political party Demosisto with Wong earlier this year. Law, 23, was elected Hong Kong's youngest legislator last month.

He urged Hong Kong Justice Secretary Rimsky Yuen, who departed Wednesday for Bangkok on a previously scheduled three-day, to raise the issue with Thai officials.

A Thai security report obtained by The Associated Press said that more than 10 police officials from the Royal Thai Police Special Branch and Immigration Police were waiting for Wong when he arrived. It said that he was questioned and not allowed to use his cellphone or computer.

China's Foreign Ministry said in a brief statement that it was aware of reports of 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."

Wong's case is sure to raise concern in Hong Kong, where residents are growing increasingly worried about Beijing overstepping its boundaries and undermining a "one-country, two systems" formula that governs its relationship with the mainland.

Many were shocked at the recent disappearances of five booksellers, including Chinese-born Gui Minhai, a naturalized Swedish citizen who went missing from his holiday home in Thailand and later turned up in mainland Chinese custody.

Refusing entry to 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, 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.

Wong was also turned back in May 2015 when trying to enter Malaysia to speak at seminars in four cities. Malaysian officials said Wong was banned from entering the country but did not explain why.

Demosisto 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 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, 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.