BEIJING (AP) — Two decades after China's crackdown on pro-democracy protests in Tiananmen Square, a group of mothers who lost their children in the uprising is calling on the country's leaders to "break the taboo" and account for the deaths.

The Tiananmen Mothers group sent an open letter to legislators calling for a full investigation, compensation to victims' families and punishment of those responsible for the military crackdown on student-led protesters.

The appeal, released Friday through the New York-based group Human Rights in China, is similar to ones made in the past by Tiananmen Mothers. But it comes just months before the 20th anniversary of the Tiananmen protests during a politically sensitive year in which the government is dealing with concerns about social stability and the ongoing economic crisis.

The letter, signed by 127 people who said their children or family members were victims, calls the military crackdown "nothing short of an unconscionable atrocity."

At the time, China's leaders deemed the peaceful protests a threat to Communist Party rule and sent tanks and troops on June 3-4, 1989, to crush them. Hundreds — possibly thousands — of people are believed to have been killed.

In the 20 years since, China has never offered a full accounting of the crackdown, which government leaders refer to as a "political disturbance." An official silence has been maintained around the incident, with nothing written in school textbooks and public discussion virtually taboo.

"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," said the letter, referring to delegates attending next week's annual legislative session of the National People's Congress.

Earlier this week, newspapers in Hong Kong reported that a leading pro-Beijing politician had condemned the crackdown on the 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.

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 in predominantly Tibetan areas in western China with the memory of last year's anti-government riots in Lhasa still fresh.

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

Police across the nation are being trained in Beijing to improve their response to public security threats in the provinces.

On Thursday night, police in Beijing took away more than 1,000 petitioners who came to seek redress for problems with local officials ahead of the legislative session, a supporter said Friday.

They were pulled from their hotel rooms by security forces and driven away in public buses, said Zhou Li, a Beijing native who helps petitioners.