A group of Chinese dissidents is reported to have signed an open letter calling on the government to publicly apologise for the violent suppression of the pro-democracy demonstrations in Beijing nine years ago.

A Hong Kong-based human rights group said the petition urged the Beijing authorities to erect a memorial to the victims.

At least twenty people are reported to have been detained in the past ten days as Chinese police step up security measures before this week's anniversary of the events in Tiananmen Square.

In Hong Kong, more than fifteen-hundred marchers have marked the anniversary and called for greater political freedom on the mainland.

The BBC correspondent in Hong Kongsays the ability of pro-democracy groups to continue to hold such demonstrations is a key test of the territory's autonomy under Chinese rule.