Victoria Park became a galaxy of candlelight last night with what was claimed to be a record crowd marking the 25th anniversary of the June 4 crackdown.
Conflict within the ranks of the democracy movement led to radicals holding an alternative rally in Tsim Sha Tsui.
But the Hong Kong Alliance in Support of Patriotic Democratic Movements in China put the number in Victoria Park at more than 180,000, well up from the 150,000 reported last year.
Watch: Hong Kong hosts China's largest -- and only -- 25th anniversary Tiananmen remembrance rally
Police put the turnout at 99,500, compared with 54,000 last year. The previous biggest turnout reported by the alliance was 180,000 in 2012.
Even as the commemoration started at 8pm, thousands of people were waiting to get into the park, with the queue stretching all the way to the Sogo department store in Causeway Bay.
Many travelled from the mainland to join the vigil, with Hong Kong - and its neighbouring Macau - the only places on Chinese soil where people can observe the anniversary in public and on such a large scale.
"Let's show our sea of lights to [President] Xi Jinping ! Fight until the end!" said alliance chairman Lee Cheuk-yan.
In contrast, Beijing was eerily quiet. Tiananmen Square and other key locations were heavily guarded by police. Only a few family members of four of the victims killed in the 1989 crackdown were allowed to visit the Wanan Cemetery where their loved ones were buried. They were closely watched by security officers.
Zhang Xianling, a founder of the Tiananmen Mothers group, said she alone was watched by at least 20 plain-clothes officers. "This shows the authorities still lack the courage to face the grievous mistake they made 25 years ago," she said.
In Hong Kong, video messages from eight exiled dissidents, including Wang Dan, Wuer Kaixi, Yan Jiaqi and Wang Juntao, were broadcast. Wang Dan said the desire for democracy lived on.
"We will certainly see the dawn of the victory if we continue to persevere," he said.
Teng Biao, a mainland civil rights lawyer, said: "I hope one day Chinese citizens will have the freedom of protest, which is a right protected by the Chinese constitution, as well as a basic human right.
"However, many human rights activists … have sacrificed their freedom trying to pursue this right. Some have even lost their lives for it."
At the close of the gathering, the alliance called on participants to join the annual July 1 pro-democracy march.
The Federation of Students protested outside the central government's liaison office in Hong Kong after the vigil.
Radical pan-democratic lawmaker Wong Yuk-man's Proletariat Political Institute and online media platform Passion Times jointly held an alternative vigil in Tsim Sha Tsui.
Organisers put the turnout at about 7,000 - much higher than they had expected - but police said the figure was nearer 3,000. Pro-Beijing group Voice of Loving Hong Kong staged its own rally - attended by about 20 people - right next to the candlelight vigil in Victoria Park. It screened a video purportedly showing that "nobody died at Tiananmen Square".
Police stepped in to keep the group and some members of the huge crowd apart, although there were scuffles.
Watch: Groups clash with pro-Beijing group ahead of Hong Kong's June 4 vigil; one arrested
Earlier in the day, government-friendly lawmakers walked out of the Legislative Council as pan-democrats observed a minute's silence, after Lee Cheuk-yan's request for formal mourning was rejected by Legco President Jasper Tsang Yok-sing.
Yesterday in Macau, over 2,000 people gathered at Senado Square to commemorate those who lost their lives in the crackdown, the Macau Daily Times reported.
Gary Cheung, Tanna Chong, Tony Cheung, Jeffie Lam, Johnny Tam and Minnie Chan