Participants held candles to remember the victims at the vigil, which turned Victoria Park's six soccer fields into an ocean of flickering light.
More than 180,000 people joined the gathering, according to organizers, while police put the crowd size at about 99,500. It was likely one of the largest turnouts for the annual event in recent years.
Democracy activists laid a wreath at a makeshift memorial as they read out the names of those who were killed in the military suppression on June 3-4, 1989, in the heart of Beijing.
Many participants at Wednesday's rally wore white headbands on which were written "June 4 25 years" and followed chants urging China's Communist Party to overturn its verdict on the protest movement. Beijing has said the protests aimed to topple the ruling party and plunge China into chaos.
Also Read
The protests remain a taboo topic in mainland China, and Beijing has never given a full accounting of what happened during the crackdown or its human toll.
But in Hong Kong, which retains Western-style civil liberties unseen on the mainland, the memory of the Tiananmen protests reinforces the widening differences with China 17 years after the territory ceased to be a British colony.