Scores of police and paramilitary troops patrolled the vast plaza in the city's heart and surrounding streets, stopping vehicles and demanding identification from passers-by.
Accompanied or otherwise monitored by police, some relatives of the crackdown's victims paid respects at cemeteries or at home, expressing frustration at being prevented from organizing public memorials.
China allows no public discussion of the events of June 3-4, 1989, when soldiers backed by tanks and armoured personnel carriers fought their way into the heart of Beijing, killing hundreds of unarmed protesters and onlookers.
Yin Min, whose 19-year-old son, Ye Weihang, was killed in the crackdown, said she wept in grief as she hugged his ashes at home in the morning.
"I told my son this morning, 'Your mother is powerless and helpless, after more than 20 years I don't even have the chance to appeal for support,'" Yin said in a phone interview from her home in Beijing, crying as she spoke.
"I looked at his ashes, I looked at his old things, and I cried bitterly," Yin said. "How has the world become like this? I don't even have one bit of power. Why must we be controlled so strictly this year?"
The government has largely ignored the relatives' demands for an admission of wrongdoing and for a complete, formal accounting of the crackdown or the number of casualties.
Near the square in Beijing, reporters were told to leave following the daily crack-of-dawn flag-raising ceremony and there were no signs of demonstrations or public commemorations. Dozens of dissidents and other critics have already been detained by police, held under house arrest or sent out of the city.
Activists say this year has seen a longer and more restrictive clampdown than usual, reflecting the increasingly conservative political atmosphere under President Xi Jinping, who took office last year.
"I regret I can't to go to the square to pay tribute, but it warms my heart that those events and those sacrifices have not been forgotten after 25 years," said veteran activist Hu Jia, who has been under house arrest for 101 days.
Yin said the authorities' monitoring of her family intensified this year, with round-the-clock police surveillance that started in April. The relatives' plans for a public memorial like ones they had previously held every five years could not be realized, she said.
"You're not only re-opening my scars, you're spreading salt and chili powder into it," Yin said she told her minders. "I said, 'You are too inhuman.'"
Authorities allowed relatives of some of those killed in the crackdown to visit their loved ones' graves, but they had to go quietly and under police escort, according to Zhang Xianling, a member of a group that campaigns for the crackdown's victims.
"The wound is still very deep. And though we might now shed fewer tears than in the past, our conviction is even stronger," said Zhang, whose son, Wang Nan, was 19 years old when he was killed in the suppression. "We must keep struggling until the end. We must pursue justice for our loved ones."
Beijing's official verdict is that the student-led protests aimed to topple the ruling Communist Party and plunge China into chaos. Protest leaders said they were seeking broader democracy and freedom, along with an end to corruption and favouritism within the party.
Activist lawyer Teng Biao said the government's repression only betrayed its frailties and fear of dissent. "Although the government appears stronger, they are more fearful, less confident and have less sense of security," Teng said from Hong Kong, where he is a visiting scholar at the city's Chinese University.
Foreign media in Beijing have been warned not to meet with dissidents or report on issues related to the anniversary. In an unusual burst of activity, the Foreign Ministry and Cabinet office held news conferences and called in Associated Press reporters for meetings Wednesday.
The Tiananmen protests remain a totem for political expression and Western-style civil liberties in Hong Kong, a former British colony that retained its own liberal social and legal systems after reverting back to Chinese rule in 1997.
The city holds an annual candlelight vigil to remember the Tiananmen victims that's attended by tens of thousands of people, with the numbers rising in recent years. Organizers said they were expecting about 150,000 people to attend Wednesday night's rally in a downtown park.
For the first time, a pro-Beijing group, the Voice of Loving Hong Kong, was planning a counter-rally at the park's entrance in support of the military crackdown, in a sign of increasing polarization in the former British colony.
An annual survey by the University of Hong Kong released Tuesday showed that support for the student-led protests has slipped, although most still thought Beijing was wrong to condemn them.
In the telephone poll of 1,005 people conducted May 17-22, 48.5 per cent of people agreed that "the Beijing students did the right thing in the June 4 incident," down from 54.1 per cent a year ago.
Pollster Robert Chung said in a statement that support for the protesters was strongest among those under 30.
"This probably reflects the demand for democracy among the younger generation," he said. The survey on Hong Kong's attitudes toward the Tiananmen Square protests, which has been conducted since 1993, has a margin of error of three percentage points.
Along with concerns about political unrest, China has recently been shaken by violence blamed on separatists from the far northwestern region of Xinjiang, adding to the increased security measures.
Associated Press writers Gillian Wong and Didi Tang in Beijing and Kelvin Chan in Hong Kong contributed to this report.