"I was illegally confined and blocked from contacting the outside world by Thai Customs and Police," Wong posted on Facebook
Wong says his passport confiscated soon after arriving in Bangkok on a flight from Hong Kong. Wong was due to take part in a panel discussion organized by student activists to mark the 40th anniversary of Thailand's Thammasart University Massacre.
"After they confiscated my passport, [they] immediately detained me at the airport detention center alone. I'm fortunate to have returned to Hong Kong," Wong said.
"It was a scary 10 plus hours. Sorry for all those who worried about me."
The activist's detention at the border was due to a letter from China, according to Thailand's most prominent group of student activists, New Democracy Movement.
Chinese authorities, however, told CNN that China respects Thai immigration policy, and did not respond to inquiries regarding the existence of a letter asking Wong be denied entry to Thailand.
Thai government spokesman Sansern Kaewkamnerd said in statement released Wednesday that there had been "no instruction or order given, pertaining Mr. Wong."
Marking university massacre
Wong had been due to meet Thai student activist, Netiwit Chotipatpaisal, at Suvarnabhumi airport early Wednesday morning but failed to appear.
Chotipatpaisal told Demosistō -- Wong's pro-democracy political party --- that the student had been detained, according to a statement on the party's Facebook page.
Chotipatpaisal says Thai airport authorities told him they had received a letter from the Chinese government, but didn't reveal its contents. Chotipatpaisal said his request to see Wong was declined.
The university gathering was already sensitive, marking a day in 1976 that saw dozens of students killed in the hands of state forces.
Thailand is currently under military rule
and the regime is widely criticized by human rights groups for being oppressive and undemocratic.
"The detention and deportation of Joshua Wong are yet another indicator that Thailand's military government will use any available means to stifle political discourse in the country," Champa Patel, Amnesty International's senior research adviser for South East Asia and the Pacific, said in a statement on Wednesday.
Earlier Wednesday, Nathan Law, president of Wong's political party Demosistō, told CNN that he was "seriously concerned" about Wong's welfare, especially in light of the recent case of Gui Minhai, a Hong Kong-based bookseller who published gossipy books about China's elite.
Gui mysteriously disappeared from Pattaya in Thailand in 2015 and appeared three months later
on state-run television in China, making an apparent confession to a fatal hit and run.
Gui was one of five booksellers who disappeared in late 2015, fueling suggestions that Beijing was cracking down on dissidents beyond Chinese borders. His daughter maintains her father was abducted.
In April, Wong announced he was forming a new political party
that would "demand self-determination" for Hong Kong.
"If we do not fight for self-determination the Chinese Communist Party will determine our future," he told CNN at the time.
In August, a magistrates' court in Hong Kong sentenced Wong to 80 hours of community service after he was convicted of taking part in a rally in front of the Central Government Offices in September 2014.
The rally sparked the two-month long Occupy Central demonstrations, also known as the Umbrella Movement, which shut down down key roads in the city.