China security has tightened Tiananmen Square with hundreds of police on Thursday to prevent any commemoration of the crackdown on pro-democracy protesters 20 years ago.

Tens of Thousands people in Hong Kong attended a candlelight vigil on Thursday night to mark the 20th anniversary.

The turnout was estimated at 150,000 people, organizers said. The figure was even higher than in 1990 when the annual vigil first began, but Hong Kong police put the estimate at 63,800.

We will never forget June 4, the crowds chanted while singing remembrance songs, waving candles and linking arms.

China has tried to whitewash the crackdown and erase any mention of the Tiananmen protests.

Two days ago, authorities blocked access to the social messaging site Twitter and online photo sharing service Flickr, as well as email provider Hotmail.

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.

China 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.

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.

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.

Many in Hong Kong acknowledge China's economic progress since 1989, but still find the memories of June 4 impossible to forget.

I know China is improving, but I hope that they will admit that they fired guns and did wrong, said Kong Choi-fung, a 44-year-old church worker who took her two children to the vigil.

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