China blanketed Tiananmen Square with police officers on Thursday, determined to prevent any commemoration of the 20th anniversary of a military crackdown on pro-democracy protesters that left hundreds dead.

Visitors to the sprawling plaza in central Beijing were stopped at checkpoints and searched, and foreign television crews firmly turned away. Uniformed and plainclothes officers outnumbered tourists.

There was no flicker of protest. Other than the intense police presence, the day passed like any other in the capital.

China’s government has tried hard over the years to obliterate the memory of the huge student-led protests that shook the Communist Party and captivated the world for weeks.

An official reacted angrily on Thursday to a call by Secretary of State Hillary Rodham Clinton for a public accounting of the incident. “The US action makes groundless accusations. We express strong dissatisfaction,” Foreign Ministry spokesman Qin Gang said.