The Chinese government hopes it will be a non-event and it has tightened security in the capital, Beijing.
Presenter: China Correspondent Stephen McDonell
Xu Youyu, Philosophy Professor; Wang Junxiu, former law student; various young people
This time 20 years ago, thousands of students sat in China's Tiananmen Square, not knowing that their 6-week long protest would come to a violent end that night. Troops had been brought in from outside Beijing because the local soldiers couldn't be relied on to shoot down the Country's best and brightest... especially people they knew.
On the night of 3rd of June and the morning of the 4th, the People's Liberation Army took back control of the streets at the point of a gun.
Most of those who died were ordinary people who tried to stop the tanks reaching the students from outside the Square. Soldiers were also killed when mobs attacked them using anything they could lay their hands on.
If the casualty figure is known to Chinese authorities, it remains a state secret.
Philosophy Professor Xu Youyu was on the Square for the last night of the protests.
"[Chinese - translated] To shoot defenceless people? He asks rhetorically. Even if I was one of China's rulers, even if I fully supported China's autocracy, even if I agreed to crack down on the students, I'd only agree to use the police to drag the students from Tiananmen Square - to use tanks and machineguns is totally unacceptable".
Xu says that, since then, though the Government has kept this event out of nearly all public discourse in mainland China... some University students in 2009 do know about it.
So we asked young people on the street what happened on 4th of June 1989.
"I'm sorry I don't know, I'd just been born," says one man.
"I was really young then," says one woman. "I don't know".
Two young women seem to know about it, saying it was to do with students. 'What did the students do,' We ask?
"The students..." one starts to say. "We're not too sure about that," the other cuts in, sorry, and they walk off.
International news reports about this anniversary, like those from the BBC and Voice of America, are being blocked in China. And, in preparation for tonight and tomorrow, the authorities have started denying foreign reporters access to Tiananmen Square.
For the class of 89 they're left to wonder whether things could have been different if they'd pulled out of the square earlier.
Wang Junxiu was a third year law student at the time.
"[Chinese - translated] You may say if we'd taken more modest measures... he says. less people would've been killed - that's possible. But how then would we have been able to promote Chinese democracy?"
Tonight the group known as "the Tiananmen Mothers" will try and hold a vigil in memory of their loved ones who were killed - they all risk being arrested