IN an act of political derring-do, an environmental group on Aug. 15 threw down a challenge to China's iron-fisted police rule - right under the nose of the late dictator Mao Zedong.
Just days before thousands of international activists converge on Beijing for the United Nations women's summit, Greenpeace staged a brief protest against Chinese nuclear-weapons testing in front of Mao's famous portrait on Tiananmen Gate, the political heart of China.
Five environmental activists unfurled a 16-foot yellow banner urging, ''Stop All Nuclear Testing.'' The names of the five major nuclear powers - the United States, Russia, France, Britain, and China - were written in English and Chinese.
No sooner was the sign opened than plainclothes police swarmed in, grabbing the sign and preventing the protesters from displaying smaller banners. They were taken away by the police.
Greenpeace, which indicated that it also distributed leaflets in Tiananmen Square, said the demonstration was intended to highlight another test of lightweight, powerful nuclear weapons expected soon in the western Chinese province of Xinjiang. China, whose last nuclear explosion was in May, has continued to test nuclear arms despite a moratorium by most of the other nuclear powers. Beijing reportedly plans four more tests before a comprehensive nuclear test ban treaty takes effect in 1996.
Through its often recklessly courageous, militant confrontations over nuclear testing, oil exploration, and wildlife preservation, Greenpeace has become one of the world's best-known environmental crusaders.
''We think they [Chinese officials] need to exercise leadership in the international community'' by ending the tests, says Greenpeace spokesman Damon Moglen.
COMING just before the women's conference, the demonstration appears to be the opening volley in what could become a battle between the security-conscious Chinese and the more than 30,000 activists, journalists, and delegates expected for the UN women's summit. The summit and a parallel forum of private advocacy groups will be held in the Beijing area in early September.
Fearing boisterous proceedings and political outbursts from the nongovernmental groups, which will include human rights and antiabortion advocates, the Chinese government switched the forum site at the last minute and relegated the meeting to a distant Beijing suburb.
Security has also been stepped up around the city, and an anticrime crackdown is under way to create ''a secure environment'' for the conference, according to official Chinese press reports.
Yet despite the high security, the Greenpeace gesture in Tiananmen Square - a sacred place of Communism and the target of antigovernment demonstrations in recent years - underscores the government's security dilemma as the women's meetings approach, Western diplomats say. In 1989, millions of Chinese demonstrated for more political openness and an end to political corruption in Tiananmen Square before the military crushed the protests in a bloody crackdown.
''This protest will likely embolden other groups [at the women's conference] to take their cause to Tiananmen Square,'' says a Western diplomat.