China's vast censorship machine does its utmost to wipe the slightest reference to the Tiananmen crackdown from books, television and the Internet, scrubbing the issue from public discussion and even from the minds of its younger generation.

In an example of George Orwell's "1984" dictum that "who controls the present controls the past", it reflects both the ruling Communist Party's immense power and its enduring sensitivity about its actions on June 3-4, 1989.

The overnight clearing of the square at the heart of Beijing, where student-led protesters had demanded reforms for seven weeks, left hundreds dead -- by some estimates more than 1,000 -- and the party isolated from its people and the world.

A third of China's population today was born afterwards, while many of those alive at the time hesitate to broach the sensitive topic -- leaving a huge swathe of those under 25 ignorant of the event.

"I don't know what you are talking about," a 20-year-old student at Peking University, one of China's most prestigious, told AFP when asked about the protests, looking slightly embarrassed.

Television, film and print media have always been under strict official control in Communist China.

Online, hundreds of millions of Chinese now have unprecedented access to information -- but only that approved by the authorities. An army of censors deletes topics deemed sensitive, even the most oblique references to the crackdown.

A Chinese equivalent of Wikipedia maintained by domestic Internet giant Baidu has no entry for the year 1989, let alone anything more specific.

On China's Twitter-like microblogging site Weibo, a long list of terms related to the June 4 crackdown are banned, including the characters for 6 and 4 strung together.

"The education system and the vast apparatus that censors the Chinese media and Internet have done such a formidable job at eliminating references to the events of 1989 that many young people are unaware of what happened or have only a faint notion of what happened," said Jeremy Goldkorn, the founder of Danwei, a Beijing-based firm that tracks Chinese media and Internet.

"The result is that many young people who do not remember 1989 themselves would need an unusual degree of curiosity to look for information about what happened."

- 'May 35' -

For censors in the know, no reference is too vague.

When the Shanghai stock market closed down 64.89 points on the 2012 anniversary -- an eerie echo of June 4, 1989 -- they blocked the term "Shanghai index" on social networks.

Last year they eliminated "big yellow duck" after an image circulated online parodying the Tank Man photo, with giant toy ducks standing in for the military vehicles blocked by a lone protester.

Web users find workarounds such as "May 35", "63 plus 1" or homonyms of banned words, though they too are eventually blacklisted.

"They are basically a mark of commemoration, like lighting up a candle somewhere even if no one understands what the reference is," said Jason Ng, a University of Toronto research fellow and author of "Blocked on Weibo".

"That means that you're still aware, you still want to remember."

The Chinese writer Ma Jian, who now lives in London, evoked the nation's collective silence in his 2008 novel "Beijing Coma", centred on the memories of a young Tiananmen demonstrator shot and left paralysed, mute and blind -- but aware.

The book is banned in China.

Chinese filmmaker Lou Ye's 2006 movie "Summer Palace", which depicts relationships against a backdrop of the protests, was shown at the Cannes festival but has never been released in his country.

Censors told him the sound and picture quality were not good enough for screening, he has said. He was banned from directing for five years.

- Duty to speak -

One group that refuses to stay silent is the Tiananmen Mothers, parents who lost children in the crackdown and every year call on authorities to give an account of what happened.

Yet Zhang Xianling, whose 19-year-old son was killed, sympathises with Chinese who do not try to learn more.

"A lot of people don't have time to know about it, or don't want to know about it, because they are busy, or want to make a living, or have to work -- this is understandable," she told AFP.

"But I believe that such a huge incident, such a huge tragedy, where so many innocent people were massacred... the truth cannot be covered up with lies forever."

Nonetheless Cui Weiping, an outspoken professor at Beijing Film Academy, says there is a duty to speak out. If silence continues, she has written, "June 4 will no longer be a crime that was committed by a small group of people, but one that we all participated in. It will become a shame on all of us".

Many of the participants at a private seminar she attended on Tiananmen three weeks ago have been detained, and she told AFP: "The situation is getting worse and worse.

"Of course, to remember is a moral obligation," she said. "Anything else is a betrayal of the people who were killed."