BEIJING, MAY 27 -- Three Chinese democracy activists have written an open letter to the country's leaders urging the release of political prisoners, in one of the boldest public expressions of dissent since the army crushed a democracy movement last June.

Hou Dejian, prominent in last year's democracy demonstrations, said in a recent interview that he, university teacher Gao Xin and economist Zhou Duo had signed the letter. "There are three of us now and we are using this letter as a thermometer to test the political heat," said Hou, a rock singer who left a successful recording career in Taiwan in 1983 to move to mainland China.

"I am not going to drive the mad dog {government} crazy," said Hou, who is making the letter public. "We want to build a new China not by blood but by reason."

The letter demands the release from prison of former university teacher and dissident Liu Xiaobo, arrested after the army crackdown in Beijing last year. "It asks the government to release all non-criminal detainees {arrested following the crackdown}," said Hou.

The letter says that while people in many socialist countries have recently enjoyed a "peaceful evolution" toward democracy, the Chinese are more repressed than at any time in the last decade.

After the crackdown, authorities launched a nationwide hunt for dissidents. Thousands are believed to have been detained without trial and some were executed. As the anniversary of the crackdown approaches, students say they are too scared to show sympathy for the hundreds of civilians who died in the army assault.

Liu and the letter's three signatories were on hunger strikes in Tiananmen Square the night of the assault. Hou fled to the Australian Embassy from where he emerged in August, but the others were arrested. Gao was freed late last year, and Zhou was released this month.