BEIJING - China smothered Tiananmen Square with police on Thursday to prevent commemoration of the crackdown on pro-democracy protesters 20 years ago, as Washington demanded Beijing account for those killed.

Tanks rolled into the square before dawn on June 4, 1989, to crush weeks of student and worker protests. The ruling Communist Party has never released a death toll and fears any public marking of the crackdown could undermine its hold on power.

China has changed dramatically in the past two decades. Market reforms have lifted hundreds of millions out of poverty and transformed China into the world's third-largest economy, making similar protests on the same scale highly unlikely today.

But wary of any sign of political dissent, Beijing has tried hard to erase any mention of the Tiananmen protests.

In a sign of Beijing's mix of confidence and caution, Tiananmen Square was open to visitors on Thursday, with hundreds of police and guards present. On the 10th anniversary of the crackdown in 1999, it was closed to the public.

Chinese crowded the square to watch the dawn flag-raising ceremony that is now a fixture of official patriotic ritual. Many were visitors from outside Beijing and appeared oblivious to the sensitive date. There were no gestures of protest.

But some people came quietly to the square to mourn.

Today is June 4, so we came here to commemorate it, said a man surnamed Wang.

Thousands in China-ruled Hong Kong attended a candlelight vigil on Thursday night to mark the anniversary.

By the start of the vigil, organizers said tens of thousands had already gathered, filling an area the size of six football pitches, with a target of 100,000 seen as possible.

In 20 years, the knot of June 4 has not been untied, said Reverend Chu Yiu-ming, one of the organizers.

CRUDE MEDDLING

The 1989 killings strained ties between Washington and Beijing and the reverberations were evident on the eve of the anniversary.

U.S. Secretary of State Hillary Clinton called on China to release all those still imprisoned in connection with the protests, to stop harassing those who took part and to begin a dialogue with the victims' families.

A China that has made enormous progress economically and is emerging to take its rightful place in global leadership 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, Clinton said in a statement.

China denounced the comments as crude meddling.

We express our strong dissatisfaction and resolute opposition, said Foreign Ministry spokesman Qin Gang.

Clinton's demands reflect views Washington has long held but represent a tougher stance on China's human rights record than Clinton has taken in her first four months in the job.

Clinton's call was echoed by Australian Prime Minister Kevin Rudd, a Mandarin-speaking former diplomat once posted in Beijing.

All people around the world were affected by those events and they still have resonance today, Rudd said.

Authorities blocked access to the social messaging site Twitter (www.twitter.com), online photo sharing service Flickr (www.flickr.com), as well as briefly to email provider Hotmail. Foreign newscasts about the anniversary have been cut.

The leaders would rather just avoid this topic, said Zhang Boshu, a philosopher in Beijing who has urged a public reckoning with the killings. They know that the 1989 crackdown, shooting their own citizens, was a terrible blow to their legitimacy.

Foreign reporters were barred from the Beijing courtyard home of late reformist leader Zhao Ziyang, in a quiet alley crawling with plainclothes police and security volunteers who sat on stools sipping tea. Security officials also tightly controlled access to Beijing universities.

Dissidents have been detained or harassed, including Zeng Jinyan, wife of detained AIDS activist Hu Jia, prompting anger from rights groups.

Some Chinese activists and intellectuals recently urged the government to repent for the killings and start on a course of political liberalization. But China's leaders have shown no appetite for such steps, often saying that top-down political control is needed to guard economic growth.

The president of Taiwan, a self-ruled island claimed by China, told Beijing to face up to the truth.

This painful period of history must be faced with courage and cannot be intentionally ducked, President Ma Ying-jeou said in a statement.

While mention of the crackdown is taboo in Chinese media, dissidents have again been trying to get the government to reassess its official verdict on the incident, which is that it was a counter-revolutionary plot.

This year's anniversary comes as the economy is slowing on the back of the global financial crisis. The government has reacted quickly, unveiling a 4 trillion yuan ($585.8 billion) stimulus package and other measures to tackle rising joblessness.

I don't think students would go to the streets to demonstrate against the Chinese government in the same way as the students of the 1980s, said Bo Zhiyue, a Chinese politics expert at Singapore's East Asian Institute.

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