By Ben Blanchard

BEIJING - Chinese police were out in force on Thursday to stop any commemoration of the crackdown on pro-democracy protesters around Tiananmen Square 20 years ago, a day after 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.

The 1989 killings severely bruised relations between Washington and Beijing, and there were echoes of those tensions 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.

The 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.

In a sign of Beijing's mix of confidence and caution, Tiananmen Square was open to visitors on Thursday, but 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.

A Reuters photographer was stopped from taking pictures and told to erase those he had taken.

Authorities blocked access to popular Internet services Twitter, online photo sharing service Flickr, 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 back
alley crawling with plainclothes police and security volunteers who sat on stools sipping tea.

Security officials also tightly controlled access to Beijing universities, checking ID cards and refusing permission
to enter for most non-students.

Anniversary to be marked in Hong Kong and Taiwan

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

Mothers of some of the dead from 1989 were prevented from leaving their homes to commemorate their children.

"We express our strongest outrage and vehement resistance to the Chinese government's barbaric conduct in brazenly violating international human rights covenants and stripping grieving kin of their rights," the Tiananmen Mothers, who want a full accounting of June 4, said in a statement.

Thousands of people in Hong Kong are expected to attend a candlelight vigil to commemorate the victims, as they do every year, and in Taiwan activists will likewise mark the anniversary.

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, eliminating jobs
especially in export-dependent coastal regions and making it harder for new graduates to find work.

The government has reacted quickly to the crisis, unveiling a 4 trillion yuan (S$846.6 billion) stimulus package and a
series of 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. -Reuters