China has detained dozens of people and launched an unprecedented security operation in central Beijing to stop anyone commemorating the 25th anniversary of an event that has been virtually wiped from the collective memory of the nation.
A quarter of a century after the People's Liberation Army turned its tanks and guns on the people and marched into Tiananmen Square on the night of June 3 and early hours of June 4, 1989, any mention of the massacre of hundreds, perhaps thousands, of people is banned from public life.
More than 50 people, including activists, lawyers, journalists and relatives of students killed in the massacre, have been detained, arrested or simply "disappeared" in recent weeks because of their efforts to commemorate the anniversary, according to human rights groups.
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Most of these people have been charged with amorphous crimes that are often used to silence critics of the regime – such as "picking quarrels and provoking troubles" – and could face several years in prison if convicted.
Some were detained after holding small private gatherings in their own homes, the first time this has happened according to activists.
The crackdown highlights how potent the traumatic events of 1989 remain in Chinese politics, and how fearful the authorities are of those they fear might use those events to call for greater liberty and political participation.
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Liu Xiaobo, the Nobel Peace Prize laureate who was a key figure in the Tiananmen Square student democracy protests, has been serving an 11-year prison sentence since 2009 for advocating peaceful political reform.
Several foreign governments and international bodies have expressed concern over the crackdown.
"We are deeply concerned by the recent arrests and detentions of a large number of peaceful human rights defenders, lawyers and intellectuals," the EU said last week.
"We reiterate the EU's call on the Chinese authorities to abide by the Universal Declaration of Human Rights . . . and to release all those imprisoned for the peaceful expression of their views. We also urge the Chinese authorities to ensure that, where needed, detainees have rapid access to independent and adequate medical care, in order to avoid a repetition of recent deaths in custody."
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There have been some calls on social media in recent weeks for ordinary Chinese citizens to "return to the square" on June 4 to commemorate the massacre.
But even such a harmless act as strolling through the square could prompt a harsh response from the authorities, who have launched a show of force in the center of the capital, with armed police and paramilitary patrols and thousands of plain-clothed officers.
Beijing is also on high alert after a series of terrorist attacks around the country, which the government blames on Muslim extremist Uighur separatists from the far west of China.
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"The response by the Chinese authorities to the 25th anniversary has been harsher than in previous years, as they persist in trying to wipe the events of 4 June from memory," said Salil Shetty, secretary-general of Amnesty International.
While any mention of the events of 1989 are banned in mainland China, crowds gather each year on the anniversary in the former British colony of Hong Kong, which enjoys greater freedoms than the rest of China.
In recent weeks some foreign journalists, most of them from global television networks, have been harassed and threatened by Chinese police and warned not to report on the anniversary.