Hong Kong's Immigration Department has been accused of operating a blacklist of mainland dissidents after a US-based activist was barred from entering the city for the fourth time.
Dr Yang Jianli had planned to attend the opening of the world's first permanent museum dedicated to the June 4, 1989 crackdown on student protests in Beijing's Tiananmen Square.
But after arriving at Chek Lap Kok airport from Taiwan on Saturday night, Yang said he went through three hours of interrogation by immigration officials before being placed on a flight back.
Watch: World's first June 4 museum to open in Hong Kong
"It proves that there is a blacklist in Hong Kong which tries to block" dissidents, Yang said of his case. "The list is not decided by Hong Kong's government but Beijing, which attempts to intervene in the city's internal affairs."
The Immigration Department said it would not comment on individual cases.
Yang was studying for a doctorate in the United States when the 1989 democracy protests began, but flew home to join the movement. He returned to the United States after the bloody crackdown to complete his education and campaign for democracy in his homeland.
Beijing refused to renew Yang's passport two years later, and jailed him for five years when he entered China illegally in 2002.
He was refused entry to Hong Kong in August 2008 and ahead of the June 4 commemoration in 2009, then again in 2011, when he wanted to join a conference on the 1911 revolution.
Yang said the June 4 museum, which had a trial opening yesterday, was important both to Hong Kong and the human rights movement on the mainland, as it was the only place on Chinese soil dedicated to illustrating the truth about the crackdown.
Lee Cheuk-yan, of museum organiser the Hong Kong Alliance in Support of Patriotic Democratic Movements in China, condemned the refusal to allow Yang in.
Yang "was part of the generation of the 1989 democracy movement and he witnessed the June 4 massacre", Lee said. The veteran activist "wants to … come and see the museum".