Hong Kong is the only location on Chinese soil to see a major commemoration, with residents gathering in Victoria Park to mark the military's brutal crushing of pro-democracy protests in central Beijing in 1989.
Hundreds -- by some estimates more than a thousand -- died after the Communist Party sent tanks to crush demonstrations at the square in the heart of Beijing, where student-led protesters had staged a peaceful seven-week sit-in to demand democratic reforms.
"This is an ongoing struggle for justice," said Richard Tsoi of the Alliance in Support of Patriotic Democratic Movements of China, which organises the vigil.
He predicted 150,000 would come.
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The US called for "an official accounting of the victims" of the 1989 crackdown Thursday, as well as the release from prison of those serving Tiananmen-related sentences.
The State Department also urged a halt to the harassment and detention of those who want to commemorate the anniversary.
Zhang Xianling, 77, whose then 19-year-old son was killed during the crackdown, said she visited his grave site in Beijing Thursday, where dozens of plainclothes security staff stood watching.
More than seven security officers remain near her apartment, with two guarding her door, she told AFP.
"Their main purpose is to stop me receiving interviews... and visiting areas deemed sensitive, such as Tiananmen Square," she said.
The Hong Kong vigil comes as tensions are high just two weeks ahead of a vote on the government's controversial election roadmap.
The election proposal goes before the legislature on June 17 and lays out a plan for the first ever public vote for Hong Kong's chief executive.