There was no immediate confirmation that the people detained were members of the now-banned Falun Gong, but they were whisked away to an unknown destination in buses just as sect protesters had been earlier in the week.
Plainclothes police and members of the paramilitary People's Armed Police circled the protesters and sealed off the square -- the political heart of China and the scene of student-led pro-democracy demonstrations that were crushed in June 1989.
The square was re-opened after a few hours.
At a stadium on the outskirts of Beijing, buses were seen carrying hundreds of sect followers who had been rounded up by police.
The passengers chanted Â"Falun Dafa, Falun DafaÂ" -- an alternative name for the sect -- as the buses pulled up to the stadium in Fengtai.
China banned the sect on Thursday, saying it was an illegal organization that cheated people and threatened social stability.
But analysts said China risked creating a self-fulfilling prophecy with its heavy-handed attack on the apolitical group.
Â"The authorities claim to see political shenanigans going on, but as far as anyone else can tell, it's a largely apolitical movement of grannies and middle-aged people doing qigong,Â" said a Beijing-based Western diplomat.
Qigong is a traditional system of meditation and breathing exercises from which Falun Gong grew.
Â"This risks really politicizing what is essentially an apolitical movement and creating a whole new community that doesn't like the Communist Party,Â" the diplomat said.
The leader of the Falun Gong called on Beijing not to treat his group as Â"enemies.Â"
Â"It is my hope that the Chinese government and its leadership will not treat the people who practice Falun Gong as enemies,Â" Li Hongzhi said in a statement posted on the Falun Gong website.
Â"The consequences would cause people to lose confidence in the government and its leadership and to be disappointed in the Chinese government,Â" the statement said.
State media denounced U.S.-based Li as Â"an evil person who has had an extremely disastrous effect on society.Â"
The 47-year-old Li says science has created an immoral world plagued by drugs, television, and rock 'n' roll music. He says followers can acquire supernatural powers and do not need doctors because they can cure themselves by practicing Falun Gong.
Thousands of sect members tried to besiege the Zhongnanhai leadership compound in Beijing and other government offices in more than 30 cities on Wednesday to protet the detention of 100 sect leaders.
Falun Gong, a form of spiritual meditation mixed with Chinese mysticism and a conservative social doctrine, says it has 100 million members. The government says it has two million.
Whatever the figure, the group's boldness and the quiet persistence of its followers has struck fear in the atheist Communist Party, which has 60 million members.
State media said people attempting to practice in public or disseminate the sect's books would be jailed.
The group first shocked the government in April, when 10,000 members staged a sit-down protest at Zhongnanhai to demand official respect for the organization.
In announcing the ban Thursday, state media called the April protestÂ"the most serious incidentÂ" since the Tiananmen Square democracy demonstrations of 1989 that troops crushed, killing hundreds.
While far greater in numbers, the Tiananmen protesters never dared, as Falun Gong believers did, to surround the Zhongnanhai leadership compound.
Since April, the Communist Party and government have tried to clean their ranks of the sect's followers and waged an anti-superstition campaign in the state media.
The United States expressed concern about China's moves against the sect.
Beijing's many parks were empty of Falun Gong groups who normally practice their morning exercises, but people practicing other forms of qigong were out as usual.
©1999 CBS Worldwide Corp. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press and Reuters contributed to this report