BEIJING — China aggressively deterred dissent in the capital on today's 20th anniversary of the crackdown on democracy activists in Tiananmen Square. But thousands turned out for a candlelight vigil in Hong Kong to mourn the hundreds, possibly thousands, of demonstrators killed.

The central government ignored calls from U.S. Secretary of State Hillary Rodham Clinton and even Taiwan's China-friendly president for Beijing to face up to the 1989 violence.

In Beijing, foreign journalists were barred from the vast square as uniformed and plainclothes police stood guard across the area, which 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 the square to cover the raising of China's national flag, which happens at dawn every day. Plainclothes officers confronted journalists on the streets surrounding the square, cursing and threatening violence against them.

The repression on the mainland contrasted starkly with Hong Kong, where organizers said 150,000 people gathered in the city's famous Victoria Park in the largest commemoration on Chinese soil. Police had no immediate crowd estimate.

A former British colony, the territory has retained its own legal system and open society since reverting to Chinese rule in 1997.

"It's time for China to take responsibility for the killings," said Kin Cheung, a 17-year-old Hong Kong student. "They need to tell the truth."

On the mainland, government censors shut down social networking and image-sharing Web sites such as Twitter and Flickr and blacked out CNN and other foreign news channels each time they aired stories about Tiananmen.

Dissidents and families of crackdown victims were confined to their homes or forced to leave Beijing, part of sweeping efforts to prevent online debate or organized commemorations of the anniversary.

"We've been under 24-hour surveillance for a week and aren't able to leave home to mourn. It's totally inhuman," said Xu Jue, whose son was 22 when he was shot in the chest by soldiers and bled to death on June 4, 1989.

Officers were also stationed outside the home of Wang Yannan, the daughter of Zhao Ziyang, the Communist Party leader deposed for sympathizing with the pro-democracy protesters, according to the Hong Kong-based Information Center for Human Rights and Democracy. Wang heads an auction firm and has never been politically active.

In a further sign of the government's intransigence, the second most-wanted student leader from 1989 was forced to return to Taiwan today after flying to the Chinese territory of Macau the day before in an attempt to return home.

Wu'er Kaixi, in exile since fleeing China after the crackdown, 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."

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.