Security is tight and access restricted to Beijing's Tiananmen Square on the 35th anniversary of the 1989 crackdown, while Hong Kong has also increased policing as activists in Taiwan and elsewhere prepared to mark the date with vigils.
Chinese tanks rolled into the square before dawn on June 4, 1989, to end weeks of student and worker protests.
Decades after the military crackdown, rights activists say the demonstrators' original goals - including a free press and freedom of speech - remain distant, and June 4 is still a taboo topic in China.
The ruling Communist Party has never released a death toll, though rights groups and witnesses say the figure could run into the thousands.
Taiwan's president Lai Ching-te said in a statement on Tuesday that "the memory of June 4th will not disappear in the torrent of history".
Lai, who was inaugurated last month as the leader of the democratic island China claims as its own, added that Taiwan would "respond to authoritarianism with freedom."
In Beijing, a Chinese foreign ministry spokesperson, Mao Ning, told reporters that Beijing "firmly opposes anyone smearing China and using this (June 4) as a pretext to interfere in China's internal affairs".
An official website for Tiananmen Square posted a notice earlier saying the tower would be closed for the entire day on Tuesday, and that those who had bought tickets for the square could get them refunded.
The official social media account of the Beijing subway network announced that an exit of Tiananmen East station would be closed from Sunday to Wednesday.
Small groups of "stability maintenance" volunteers - retirees with red armbands - have been keeping watch at neighbourhoods in central Beijing since last week. Guards have also been stationed on pedestrian bridges, a regular practice during politically sensitive periods.
On Chinese social media platforms including WeChat and Douyin, users were unable to change their profile photos, according to online posts and Reuters tests.
"Thirty-five years have passed, and the authorities remain silent. All that can be seen on the internet is 'A Concise History of the Communist Party of China', which says that a tragic incident was caused by the student movement in 1989," wrote the Tiananmen Mothers, a group of mostly China-based survivors and families of the victims of the Tiananmen crackdown.
"We cannot accept or tolerate such statements that ignore the facts."
In China-ruled Hong Kong, police officers tightened security around downtown Victoria Park, where large Tuesday candlelight vigils had earlier been held annually before tougher new national security laws came into force in recent years.
Performance artist Sanmu Chen was taken away on Monday night by police as he attempted a mime performance near a police van. Chen was later released.
Last Tuesday, Hong Kong police arrested six people for sedition under a new national security law enacted this year.
Australia's Foreign Minister Penny Wong highlighted the brutal force used against student protesters 35 years ago and said her country remained concerned about China's ongoing restrictions on individual rights.
"We call on China to cease suppression of freedoms of expression, assembly, media and civil society and to release those detained for peacefully expressing their political views," she said in a statement on Tuesday.
