BEIJING (AP) — A Taiwanese academic says he was turned away by Hong Kong immigration officials after he flew to the territory to speak at a conference commemorating the 25th anniversary of the Tiananmen Square pro-democracy movement.

Tseng Chien-yuan said he was told Friday morning that the central government in Beijing had issued an order banning him from the city. Hong Kong is a former British colony that retains its own laws and civil liberties, but mainland China controls immigration and external security.

Tseng’s rejection underscores China’s extreme sensitivity toward commemorations of the bloody crushing of the 1989 student-led movement centered on Beijing’s Tiananmen Square.

Hong Kong Secretary of Security Lai Tung-kwok told reporters he couldn’t comment on individual cases but that officers had to turn away those who didn’t meet “relevant requirements.”