Police in China's capital, Beijing, on Thursday ringed Tiananmen Square on the 20th anniversary of the violent suppression of student-led protests.
The clampdown comes as international pressure mounts for China to issue a full account of what happened in 1989.
Hundreds, possibly thousands, died in the massacre. Public marking of the deaths is still not tolerated in China and discussion of the issue is banned.
The Tiananmen protests started in Spring 1989, when students began calling for greater democracy and anti-corruption measures.
Tanks rolled into Tiananmen Square before dawn on June 4, 1989 to crush weeks of student and worker protests.
The ruling Communist Party has never held an official inquiry into what happened in and around the square 20 years ago, or an official death toll.
China said the United States had made "groundless accusations" after Washington called for a listing of those killed, the BBC reports.
"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," US Secretary of State Hillary Clinton said in a statement released on Wednesday.
She said the Beijing government need to "provide a public accounting of those killed, detained or missing, both to learn and to heal".
China expressed its "strong dissatisfaction" with Mrs Clinton's comments.
"The US remarks, which disregard the facts, make groundless accusations against the Chinese government," foreign ministry spokesman Qin Gang said.
"We urge the US to put aside its political prejudices and correct its mistakes so as to refrain from undermining bilateral relations.
"On the political incident that took place in the 1980s, the party and the government have already reached a conclusion," he said.
Tight security
There is very tight security in Beijing, with police stopping foreign journalists reaching the square.
Police vans were stationed at the side of the Forbidden City, near the square, while police and paramilitary forces patrolled through crowds of tourists, and examined visitors' bags and papers.
Uniformed and plain-clothed officials stopped television crews filming the daily flag-raising ceremony at dawn.
Many dissidents say they have been told to leave Beijing or are confined to their homes, and plain-clothes police are at the gates of the city's universities to prevent any commemorative events.
Hong Kong, with its separate legal and political system and where freedom of expression is guaranteed, is the only Chinese territory due to hold a commemoration, with a candlelight vigil scheduled for Thursday.
