An exiled protest leader - famous for publicly haranguing one of China's top leaders 20 years ago - was also blocked from returning home to confront officials over what he called the "June 4 massacre."
Uniformed and plainclothes police stood guard across the vast plaza that was the epicenter of the student-led movement that was crushed by the military on the night of June 3-4, 1989.
CBS News correspondent Barry Petersen reports that Tiananmen Square was open for tourists Thursday, but not to foreign journalists, including himself.
"We suddenly needed special permission, and authorities said that might take days," reports Petersen. "China's leaders want no publicity about this anniversary."
Plain clothes officers aggressively confronted journalists on the streets surrounding the square, cursing and threatening violence against them.
The extraordinary security moves come after government censors shut down social networking and image-sharing Web sites such as Twitter and Flickr, and blacked out foreign news channels such as CNN each time they aired stories about Tiananmen. Dissidents were confined to their homes or forced to leave Beijing, part of sweeping efforts to prevent online debate or organized commemorations of the anniversary.
In another sign of the government's unwavering hard-line stance toward the protests, the second most-wanted student leader from 1989 said he had been denied entry to the southern Chinese territory of Macau.
Wu'er Kaixi, in exile since fleeing China after the crackdown, traveled to Macau on Wednesday to turn himself in to authorities in a bid to return home. He told The Associated Press by phone he was held overnight at the Macau airport's detention center and that being denied entry on the Tiananmen anniversary was a "tragedy."
He returned to Taiwan later Thursday.
Wu'er rose to fame in 1989 as a pajama-clad hunger striker yelling at then-premier Li Peng at a televised meeting during the protests. Named No. 2 on the government's list of 21 most-wanted student leaders after the crackdown, he escaped and now lives in exile in the self-ruled island of Taiwan. An attempt to return home in 2004 was rebuffed when he was deported from the Chinese territory of Hong Kong.
Wu'er said in a statement issued through a friend that he wants to turn himself in to the Chinese authorities so he can visit his parents - who haven't been allowed to leave China.
The student leader who topped the most-wanted list, Wang Dan, was jailed for seven years before being expelled to the United States in 1998.
In Washington, U.S. Secretary of State Hillary Rodham Clinton said in a statement Wednesday that China, as an emerging global power, "should examine openly the darker events of its past and provide a public accounting of those killed, detained or missing, both to learn and to heal."
In a statement marking the anniversary, Taiwanese President Ma Ying-jeou urged China to lift the taboo on discussing the crackdown.
"This painful chapter in history must be faced. Pretending it never happened is not an option," Ma said in a statement issued Thursday.
Chinese Foreign Ministry spokesman attacked Clinton's comments as a "gross interference in China's internal affairs."
"We urge the U.S. to put aside its political prejudice and correct its wrongdoing and refrain from disrupting or underming bilateral relations," Qin Gang said in response to a question at a regularly scheduled news brifing.
Qin refused to comment on the security measures - or even acknowledge they were in place.
"Today is like any other day, stable," he said.
Beijing 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. Young Chinese know little about the events, having grown up in a generation that has largely eschewed politics in favor of raw nationalism, wealth acquisition, and individual pursuits.
Authorities have been tightening surveillance of China's dissident community ahead of the anniversary, with some leading writers already under close watch or house arrest for months.
Ding Zilin, a retired professor and advocate for Tiananmen victims, said by telephone that a dozen officers have been blocking her and her husband from leaving their Beijing apartment.
In contrast to the repression on the mainland, tens of thousands of people were expected to attend an annual candlelight vigil in the former British colony of Hong Kong, which has maintained its own legal system and open society since reverting to Chinese rule in 1997.