A massive turnout filled the former British colony's Victoria Park in an annual act of remembrance for the hundreds, perhaps thousands, of people killed in the June 3-4 onslaught in Beijing in 1989.
In Beijing, police blocked the gate of a cemetery housing victims of the crackdown on pro-democracy demonstrators as part of a sweeping annual effort to bar commemorations.
In a narrow street near Beijing's Forbidden City, security personnel patrolled outside the former house of Zhao Ziyang, the former communist party secretary who was purged and held under house arrest for perceived sympathy with the protesters.
Hong Kong and Macau, which reverted to Beijing's rule in the late 1990s but have semi-autonomous status, are the only places in China where the brutal military intervention is openly marked.
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The event has largely been expunged from official Chinese history, but Victoria Park was transformed Tuesday into a sea of demonstrators holding candles.
"Vindicate June Fourth!" protesters shouted. "We will never forget".
"The candlelight vigil tonight has an additional meaning of not just condemning the massacre 24 years ago but also condemning the suppression today (in mainland China)," Lee Cheuk-Yan, chairman of protest organisers the Hong Kong Alliance in Support of Patriotic Democratic Movements of China, told AFP.
Organisers had said they expect 150,000 people to attend the event including an increasing number of mainland Chinese. No estimate of the crowd size was immediately available.
"I hope the next generation will not have to suffer the red terror, " 42-year-old Pan Xidian from the southern Chinese city of Xiamen told AFP.
"We have not given up," the construction worker said, adding that it he was very thankful for Hong Kongers' support.
Pan had travelled to Hong Kong for the first time to commemorate the crackdown that ended weeks of nationwide democracy protests.
Independent observers tallied more than 1,000 dead in Beijing, without including victims elsewhere.