Ten years after suppressing student-led protests for freedom, Chinese security forces cordoned off Tiananmen Square again today after they detained 200 demonstrators who were protesting a massive crackdown against a popular sect.

As a barrage of propaganda and police action against the Falun Gong group continued today, some observers questioned why China had launched such a extensive campaign--the largest since 1989. Included in the propaganda were a documentary shown to each of China's 2.5 million soldiers, classes for all 60 million Communist Party members and self-criticism sessions in 15 major government bodies, involving millions more people.

Thousands of practitioners throughout the country have been detained in the past two days in a crackdown that began with the arrest of key Falun Gong organizers. Protests erupted in at least 30 cities Wednesday as the police action began.

"Aren't they being too serious?" said one Beijing-based artist. "I mean, they're moving against old ladies and the unemployed."

The party has decided to take on an organization with no distinct political program for reasons both personal and political. These factors cut to the heart of the challenges facing China's Communist Party as it attempts to steer the country into the 21st century, and they underscore the party's fear that it has lost the hearts and minds of the Chinese people.

"The Falun Gong incident is the most serious political incident since the June 4 political turbulence in 1989," said Wang Zhaoguo, head of the party's United Front Work Department. "Increase your understanding and understand the dangers," blared a front-page editorial in the party's newspaper, the People's Daily.

China outlawed Falun Gong on Thursday, calling for a "serious political struggle" against the group. Led by a former soldier, Li Hongzhi, the sect combines Buddhist teachings and predictions about the end of the world with meditation and martial arts as a prescription for physical and spiritual well-being. Most of its practitioners are retired or unemployed.

In the capital this morning, followers of Falun Gong were conspicuously absent from the diverse crowd of arm twirlers, deep breathers and martial art practitioners who assembled outside the gates of the National Art Museum for their daily exercise rituals. In their place were dozens of uniformed police officers on alert amid the ambling, exercising masses.

In a statement from his headquarters in New York, Li denied that his group had "any particular organization, let alone any political objectives." He also warned that a continued crackdown "would cause people to lose confidence in the government and its leadership and to be disappointed in the Chinese government."

One of the reasons for this crackdown is the Communist Party's unwillingness to tolerate organizations outside its control. Coupled with that is the party's fear that its members are seeking spiritual succor someplace other than at the fountain of Chinese Communist theory. Twenty years of economic reforms and contact with the West have shaken most people's belief in communism. People these days acknowledge that they join the party not for ideological reasons, but for a leg up in society.

But throughout the day, the state-run media stressed the necessity to resume studying Marxism and dialectical materialism--subjects that have been all but ignored during the past 20 years.

A Chinese scholar watching the show could not repress a laugh. "This is absurd," he said. "Completely absurd."

From the moment the attack began Thursday, China's state media has emphasized that Falun's intricate and disciplined nationwide structure constituted a threat. The media also highlighted the fact that many party members, including officials from ministries in Beijing, were followers of Falun Gong.

"Now that we have seen the harmful nature of Falun Gong, we will draw a clear line of demarcation. . . . and will never participate in any Falun Gong activities again," one former practitioner was quoted as saying.

Another reason for the crackdown is that party leaders, facing a weakening economy and serious social strains, appear to believe they can use the campaign as a way to unite their fractious membership.

Zeng Qinghong, a close ally of President Jiang Zemin and a top party official, was quoted as saying the crackdown "is of great significance for building the party" and enhancing its "cohesive and combat strength."

A final reason for moving against Falun Gong is personal. Chinese sources noted that Jiang was deeply angered by the demonstration held on April 25 when more than 10,000 Falun followers took the security services by surprise and surrounded Communist Party headquarters here to demand that the party grant Falun Gong legal status. "Jiang was furious. And no one knew the protest was coming," one source said. "So he ordered us to get back at these people."

CAPTION: Chinese military police march in a park opposite Beijing's Tiananmen Square, where 200 protesters were detained.