BEIJING - Police saturated Tiananmen Square with security today, the 20th anniversary of the bloody crackdown on pro-democracy activists, and an exiled protest leader was blocked from returning home to confront Chinese officials over what he called the "June 4 massacre."

Foreign journalists were barred from the vast square as both uniformed and plainclothes police fanned out across the plaza that had been the epicenter of the student-led movement that was crushed by the military on the night of June 3-4, 1989.

The square was closed yesterday for a welcoming ceremony for the prime minister of Malaysia and had not been reopened as of midnight. Tiananmen Square is usually closed only temporarily during important events such as the opening of the annual legislative session.

Authorities also shut down social-networking and image-sharing Web sites such as Twitter and Flickr, and authorities confined dissidents to their homes or forced them to leave Beijing, part of sweeping efforts to prevent online debate or organized commemorations of the anniversary.

In a further sign of the government's unwavering hard-line stance toward the protests, the second-most-wanted student leader from 1989 said yesterday he had been denied entry to the southern Chinese territory of Macau.

Wu'er Kaixi, who has been in exile since fleeing China after the crackdown, traveled to Macau to turn himself in to authorities in a bid to return home. Immigration officers pulled him aside and demanded he fly back to Taiwan, something he vowed to resist.

Wu'er said by phone that he was being detained in a small room guarded by a lone official at the Macau airport's immigration offices.

"If they disagree with my behavior, they can arrest me. I can accept that," he said. "But I won't let them deport me."

Wu'er rose to fame in 1989 as a pajama-clad hunger striker haranguing 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 has lived in exile in the self-ruled island of Taiwan, where he has worked as a businessman and political commentator. An effort 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 have not been allowed to visit him in Taiwan - and engage the government in a public dialogue about Tiananmen through his court trial.

He said he did nothing wrong in the 1989 protests. "I want to reassert here the Chinese government bears complete and undeniable moral, political and legal responsibility for the tragedy that happened in China in 1989," his statement said.

The student leader who topped China's most-wanted list, Wang Dan, was jailed for seven years after the crackdown before being expelled to the United States in 1998. He was in Taiwan this week to attend commemorations.

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.

There were no signs of efforts to mark the protests in mainland China, where the government squelches all discussion of the events.

As in past years, foreign media reports on issues related to the protests were blocked. Journalists trying to film on the square or interview dissidents in recent days have been detained for several hours on apparently trumped-up charges of creating disturbances, the Foreign Correspondents Club of China said.