The BBC's James Reynolds is blocked from filming near Tiananmen Square
Chinese police have ringed Tiananmen Square, to prevent people marking the 20th anniversary of the massacre.
The clampdown came as China angrily rejected calls for a review of the 1989 crackdown in which hundreds, possibly thousands, of people were killed.
US Secretary of State Hillary Clinton urged Beijing to examine the "darker events of its past".
But Chinese foreign ministry spokesman Qin Gang said Mrs Clinton had made "groundless accusations".
Open debate about the Tiananmen killings is forbidden in mainland China, and the government has never held an official inquiry.
In a statement released on Wednesday, Mrs Clinton said Beijing needed to "provide a public accounting of those killed, detained or missing, both to learn and to heal".
China expressed its "strong dissatisfaction" with her comments.
"The US remarks are groundless accusations against the Chinese government and in contravention of the fundamental norms governing international relations, as well as a gross interference in China's internal affairs," said foreign ministry spokesman Qin Gang.
"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.
'Chance to reflect'
There is very tight security in Beijing, with police stopping foreign journalists reaching the square.
Uniformed and plain-clothed officials even stopped TV crews filming the daily flag-raising ceremony at dawn.
China... should examine openly the darker events of its past
Many dissidents say they have been told to leave Beijing or are confined to their homes, and plain-clothed police are at the gates of the city's universities to prevent any commemorative events.
One journalist who has reported on several Tiananmen anniversaries told the BBC that the security this time was tighter than he had ever seen - even during the first anniversary of the massacre in 1990.
Even in Hong Kong, where freedom of expression is guaranteed under law, some dissidents have been denied entry to mark the anniversary.
Tens of thousands of people gathered for a candlelight vigil in the territory's Victoria Park late on Thursday.
Hong Kong is the only Chinese territory where Tiananmen commemorations are permitted.
Mrs Clinton said in her statement that the anniversary was a chance for China to "reflect upon the meaning of the events that preceded that day".
"China can honour the memory of that day by moving to give the rule of law, protection of internationally-recognised human rights and democratic development the same priority it has given to economic reform," she added.
Mrs Clinton's views were echoed by British Foreign Secretary David Miliband, who said that while China had made "big improvements in economic and social rights" since the Tiananmen crackdown, progress on political and civil rights had been "far slower".
The statement from Mrs Clinton represents a tougher line than the Obama administration has taken so far when addressing China's human rights record, the BBC's Kim Ghattas reports from Washington.
She says Mrs Clinton had disappointed activists at the start of her tenure by saying human rights should not interfere in discussions with China about other issues, like climate change and North Korea.
Michael Bristow, BBC reporter in Beijing
At first glance, it looked like any other day on Tiananmen Square this morning.
The square was full of people, including those on Chinese tour groups. They were easily identifiable by their identical baseball hats.
But a closer look revealed it was no ordinary day.
The Tiananmen protests started in April 1989, when students began calling for greater democracy and anti-corruption measures.
After weeks of protests, troops moved in on the night of 3-4 June 1989.
One of the most prominent Tiananmen protest leaders, Wu'er Kaixi, has returned to his home in Taiwan after being turned away from Macau where he was trying to enter mainland China.
"I'm sad the dream is not yet realised, that justice has not been done, that I cannot see my parents [whom he last saw before he fled China in 1989]. I'm sad and angry," he told the BBC's Cindy Sui in Taiwan.
Wu'er Kaixi gained national attention as a pyjama-clad hunger striker interrupting then-Premier Li Peng during a televised meeting before the violent crackdown.
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