BEIJING — Mainland China remained quiet Thursday on the 20th anniversary of the bloody Tiananmen Square crackdown, while tens of thousands of people staged a protest in Hong Kong.

Beijing, the capital, was on virtual lockdown. Key foreign news Web sites were blocked, dissidents were placed under house arrest, and police blanketed the vast square where a still-undetermined number of pro-democracy activists were killed in a violent clash with the military June 4, 1989. Journalists were kept away from the scene.

Several foreign governments called on Beijing this week to revisit its policy of ignoring the crackdown. Secretary of State Hillary Clinton said in a statement Wednesday that China “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.”

On Tuesday, Congress urged China to agree to a U.N.-backed inquiry into the crackdown, and House Speaker Nancy Pelosi, D-Calif., said she had directly petitioned President Hu Jintao to free the estimated 30 people still being held for participating in the protests.

German Chancellor Angela Merkel commented in a speech Thursday that “June 4, 1989 … marked a terrible sacrifice in Tiananmen Square.”

Taiwanese President Ma Ying-jeou, who has pushed for closer ties with the mainland, said: “This painful chapter in history must be faced. Pretending it never happened is not an option.”

Over the years, Beijing has taken a two-pronged approach to the massacre. Domestically, the incident is ignored in history books, and discussion about it is banned to the point that many young people know nothing of what happened. In arguments directed to the international community, Beijing has said the crackdown was necessary to ensure social stability, which it says was a precondition for the market-driven changes that have since transformed China into the world’s third-largest economy.

On Thursday, Foreign Ministry spokesman Qin Gang limited his remarks to a sentence: “On the political incident that took place in the 1980s, the party and the government have already reached a conclusion.”

In the weeks before the anniversary, authorities erased most traces of the massacre from the capital. Twitter and other Internet services that people could have used to coordinate gatherings were blocked, as were news Web sites such as CNN and the BBC.