BEIJING - A group of mothers whose children were killed in the 1989 protests in Tiananmen Square have renewed calls to China's leaders for a full investigation as the 20th anniversary approaches.

The Tiananmen Mothers -- a group of mothers and relatives who lost family in the June 4 pro-democracy crackdown -- has issued similar appeals before, but this one comes in a politically sensitive year where the government is scrambling to deal with concerns about social stability amid the economic crisis.

The open letter made public Friday called for an official investigation, compensation to the victims' families and punishment for those responsible.

It also urged Chinese leaders to "break the taboo" of the demonstrations and seek the truth behind the military crackdown against student-led protesters.

China's leaders sent tanks and troops to crush the peaceful protests on June 3-4, which were deemed a threat to Communist Party rule. Hundreds -- possibly thousands -- of people were believed to have been killed. The government has never allowed for a full accounting of the events and is sensitive to any criticism of its handling of the crisis.

"This will require each deputy to demonstrate extraordinary courage and resourcefulness, political courage and wisdom, to break the taboo and face head-on the unspeakable tragedy that took place 20 years ago and resolve 'June Fourth' with the truth," said the letter, referring to delegates to the annual legislature, which opens March 5.

The activist group has released a version of the letter every year for two decades ahead of the annual session of the National People's Congress.

Earlier this week, newspapers in Hong Kong reported that a leading pro-Beijing politician had condemned China's crackdown on the 1989 student protests -- a rare departure from the central government's official stance.

"Suppressing students was surely wrong," Tsang Yok-sing, who serves as president of Hong Kong's legislature, told university students, the South China Morning Post reported.

Tsang's party, the Democratic Alliance for the Betterment and Progress of Hong Kong, is seen as a staunch ally of Beijing and rarely criticizes the central government.

But this year is a particularly sensitive time for China's leadership, with several volatile anniversaries that could test the country, including next month's 50th anniversary of a failed Tibetan uprising. Security has already been stepped up in Tibet and predominantly Tibetan areas in western China with the memory of last year's anti-government riots in Lhasa still fresh.

The government has also been wary of the potential for mass social unrest as the continuing economic downturn pushed more than 20 million migrant workers out of jobs in recent months. Thousands of recently unemployed have protested factory shutdowns and demanded back pay in several Chinese cities.

The petition, released through the New York-based Human Rights in China group, was signed by 127 names of people who said their children or family members were victims of the crackdown.