In an open letter to Chinese President Jiang Zemin and Prime Minister Zhu Rongji, eight dissidents from northeast China said the government had been "absolutely wrong" to crush the seven weeks of protests centred on the square in Beijing.

Police stop a woman for a security check

They called for the release of political prisoners, for political reforms and democracy, and for compensation for victims' families and the thousands of people imprisoned in an ensuing nationwide crackdown.

The letter, drafted by dissident Liang Liwei and signed by the seven others, also urged the government to reverse its verdict that the demonstrations were a "counter-revolutionary rebellion" that had to be quelled.

Massacre

Troops backed by tanks and armoured personnel carriers shot their way through the Chinese capital on the night of June 3-4 1989, killing hundreds, possibly thousands, and ended the protests that sometimes drew crowds of more than a million.

The government has never allowed an investigation or a full accounting of the dead and injured.

To prevent commemorations, authorities stepped up security and extra military police patrolled on Saturday in front of Tiananmen gate, which overlooks the square in central Beijing.

Phone cut off

Veteran Beijing dissident Ren Wanding said police visited him on Friday and "told me not to carry out any activities".

But that did not stop him from issuing a statement that paid tribute to those "who died for a just cause".

Friends of Ding Zilin, a prominent campaigner for victims of the crackdown whose 17-year-old son was killed, said plainclothes police were stationed outside her house and that her phone had been cut off.

Ding planned to light incense and "silently remember her child," said Zhang Xianling, who visited Ms Ding on Friday.

Ms Zhang, who also lost a 19-year-old son, said she placed flowers under his portrait at home and would visit his grave.