A massive police presence ringed China's iconic Tiananmen Square on Thursday, the 20th anniversary of the bloody crackdown on pro-democracy activists, as the government continued an overwhelming drive to muzzle dissent and block commemorations.

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."

Foreign journalists were barred from the vast square as 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.

Security officials checking passports also blocked foreign TV camera operators and photographers from entering covering the raising of China's national flag, which happens at dawn every day.

The heavy security moves come after government censors shut down social networking and image-sharing Web sites such as Twitter and Flickr, blacked out CNN when it airs stories on 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 spent the night at the Macau airport's detention center, and immigration officials planned to deport him to Taiwan on Thursday.

The denial of entry on the Tiananmen anniversary was a "tragedy," Wu'er said.

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."

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 nationalism and economic development.

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.

"They won't even allow me to go out and buy vegetables," said Ding, whose teenage son was killed in the crackdown. "They've been so ruthless to us that I am utterly infuriated," she said.

The blocking of social networking sites marked a new chapter in China's attempts to muzzle dissent and control information, showing the burgeoning influence of such technology among young Chinese.

Authorities targeted message boards on more than 6,000 Web sites affiliated with colleges and universities, along with Chinese mini-blogging site Fanfou and video sharing site VeryCD. Notices on their home pages said they would be closed through Saturday for "technical maintenance." The video site YouTube has been blocked in China since March.

Jason Khoury, spokesman for Yahoo, which owns Flickr, said no explanation had been given and the company believed the restrictions were "inconsistent with the right to freedom of expression." Officials from Twitter did not comment.

In Hong Kong, where the anniversary is openly commemorated, a second dissident who took part in the 1989 events was denied entry to the territory. U.S. Consulate General spokesman Dale Kreisher said the decision to deport Xiang Xiaoji, an American citizen, was "particularly regrettable in light of Hong Kong's well-known reputation as an open society."

Xiang had planned to attend Hong Kong's annual candlelight vigil for victims of the crackdown.