For decades, Hong Kong was the only place in China where the victims of the 1989 military crackdown on pro-democracy activists at Tiananmen Square in Beijing could be publicly mourned in a candlelight vigil. This year, Hong Kong is notable for all the ways it is being made to forget the 1989 massacre.

In the days before the June 4 anniversary on Sunday, even small shops that displayed items alluding to the crackdown were closely monitored, receiving multiple visits from the police. Over the weekend, thousands of officers patrolled the streets in the Causeway Bay district, where the vigil was normally held, and set up tents where they searched people suspected of trying to mourn. They arrested four people for committing “acts with seditious intention,” and detained four others.

Zhou Fengsuo, a student leader in the Tiananmen Square protest movement, said that Hong Kong is now under the same “despotic rule” as the mainland.

“Back in 1989, we did not realize the mission of a democratic China,” said Mr. Zhou, now the executive director of Human Rights in China, a New York advocacy group. “Afterward, Hong Kong protests faced the same suppression, the same vilification and erasure of memories.”