Tens of thousands of people pack Hong Kong's Victoria Park for a candlelight vigil on Thursday. They were marking the 20th anniversary of the Chinese military's crackdown on pro-democracy activists in Tiananmen Square in Beijing. (Vincent Yu/Associated Press)

Chinese police aggressively deterred dissent on Thursday's 20th anniversary of the crackdown on democracy activists in Tiananmen Square, amid calls by Hillary Clinton and even Taiwan's China-friendly president for Beijing to face up to the 1989 violence.

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 square in Beijing as uniformed and plainclothes police stood guard across the vast plaza that was the epicentre of the student-led movement that was crushed by the military on the night of June 3-4, 1989. The exact death toll has never been released but the activist community says hundreds died in the crackdown.

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.

Paramilitary soldiers patrol Thursday near the Tiananmen Gate, on the northern end of Tiananmen Square. (Elizabeth Dalziel/Associated Press)

Plainclothes officers aggressively 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. 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."

The heavy security on the mainland comes after government censors shut down social networking and image-sharing websites 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.

Former student leader denied entry

In another sign of the government's unwavering hardline 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.

A man walks in Hong Kong's Victoria Park on Thursday. The poster behind him shows an iconic image from the 1989 Tiananmen Square protests, of a lone activist blocking the advance of a column of tanks. (Vincent Yu/Associated Press)

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

'Painful chapter in history must be faced'

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 Qin Gang 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 undermining bilateral relations," Qin said in response to a question at a regularly scheduled news briefing.

Qin refused to comment on the security measures — or even acknowledge they were in place. "Today is like any other day, stable," he said.

Dissidents under surveillance

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 favour 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.