Exiled former student leader Chai Ling, meanwhile, issued a rare public statement before Thursday's anniversary of the bloody crackdown, calling for the release of political prisoners, an independent investigation into the events and permission for former student leaders to return home.
"The current generation of leaders who bear no responsibility should have the courage to overturn the verdicts" on the protests, said Chai, in a statement distributed by the Hong Kong-based Information Center for Human Rights and Democracy.
China has never allowed an independent investigation into the military's crushing of the protests, in which possibly thousands of students, activists and ordinary citizens were killed. The subject remains taboo on the mainland, with officials routinely countering questions about Tiananmen with remarks on how much China has developed and prospered in the years since.
"The party and the government long ago reached a conclusion about the political incident that took place at the end of the 1980s and related issues," Foreign Ministry spokesman Qin Gang said Tuesday at a regularly scheduled news conference.
The government refers to the protests as "counterrevolutionary" riots.
Government Internet monitors have shut down message boards on more than 6,000 Web sites affiliated with colleges and universities, apparently to head-off any discussion of the 1989 events, the Hong Kong-based Information Center for Human Rights and Democracy said.
Numerous blogs maintained by government critics such as avant garde artist Ai Weiwei have been blocked and popular social media sites such as Twitter and photo-sharing site Flikr could not be accessed within China on Tuesday. Video-sharing site YouTube has been blocked for months.
Authorities have also been steadily tightening surveillance over China's dissident community ahead of this year's anniversary, with some leading writers already under house arrest for months.
In recent days, Bao Tong, former secretary to Zhao Ziyang, the Communist Party leader deposed for sympathizing with the 1989 protesters, was removed from Beijing by security agents and taken to his home province of Zhejiang, according to the Hong Kong center.
The phone at the 76-year-old Bao's Beijing home was answered by a woman who said she was a maid and confirmed that Bao was in Zhejiang and would not return for another week. She refused to say when he left Beijing or provide contact numbers for him in Zhejiang.
Phones at the home of retired professor Ding Zilin, an advocate for Tiananmen victims whose teenage son was killed in the crackdown, rang busy all Tuesday. Ding had said earlier that security agents "strongly suggested" she and her husband leave the capital during the anniversary.
Elsewhere, in the city of Taizhou in Zhejiang province, former educator Wu Gaoxing was taken from his home by agents on Saturday, shortly after the publication of a letter he had co-signed complaining about economic discrimination against dissidents, the New York-based Human Rights in China said in a news release.
The letter, addressed to President Hu Jintao and other top communist leaders, said former political prisoners are unable to find steady jobs and are deprived of medical benefits and pensions.
"If we get sick, we can only wait to die, and all this just because 20 years ago we were sentenced for political reasons," said the letter. The human rights group published the letter online at Wu's request, and an English translation was posted on the group's Web site.
Wu was among the hundreds detained or imprisoned in the Tiananmen crackdown and was given a two-year sentence for having organized support for the protesters.
Calls to Wu's mobile phone were met with a message saying it had been turned off, while phones at Taizhou State Security Bureau rang unanswered.
Despite China's official silence, the crackdown remains a major topic for human rights groups and pro-democracy supporters in the Chinese-ruled Hong Kong autonomous region, where this year's June 4 vigil is expected to draw tens of thousands.
Overseas monitoring groups estimate that 30 men remain in prison on charges related to the protest, and Amnesty International issued an open letter this week to China's top legislator, Wu Bangguo, calling for their release.
"The Chinese authorities should immediately release these prisoners as a first step toward accountability," the letter said.
Many overseas-based former student leaders have been blocked from returning, including Chai, who said in her statement that she saw only a slight chance of her being allowed to come home.