Joshua Wong, a student activist and spokesperson for the group Demosistō, was invited to speak at Chulalongkorn University in Bangkok. He was arrested at the airport and faced deportation.
Wong told media that he was not given an explanation by Thai authorities as to why he was sent back. One officer showed him a document that was marked "blacklist."
"As a Hong Kong person, I did not expect that even if I do not enter mainland China, I would be inside a foreign detention center, detained by other police. This is unbelievable," he said.
Colonel Pruthipong Prayoonsiri, deputy commander of airport immigration police, said that China sent a letter to the Thai government to "request cooperation to deny entry" of Wong.
Wong was responsible for organizing street protests in Hong Kong in 2014. His political party stands for democracy and the famous "Umbrella Movement".
Prime Minister Prayuth Chan-ocha said that Wong's deportation is an issue with China and not a matter for Thailand.
He said, "Why he was sent back is China's issue."
The Thai student activist who was supposed to meet Wong, Netiwit Chotipatpaisal, notified the party that Wong has been detained at Suvarnabhumi airport. Demosistō posted several tweets regarding the issue.
The post stated, "According to Chotipatpaisal, the Thai authorities have received a letter from the Chinese government earlier regarding Wong's visit. His request to see Wong, who is still currently in custody, has also been declined."
"Demosistō strongly condemns the Thai government for unreasonably limiting Wong's freedom and a right to entry, and requests the immediate release of Wong. In the meantime, we request the Hong Kong immigration department's assistance in assuring Wong's safety," the tweet stated.
The political party's secretary general, Agnes Chow, said, "We believe that the Chinese government is using all ways to stop us spreading democratic ideas from Hong Kong to other countries. But bigger suppression makes a stronger fight."