China blocked internet access to Tiananmen-related search terms and rounded up hundreds of activists in the capital Beijing marking 23 years since the bloody June 4, 1989 crackdown.
The BBC reported that a small group of elderly men who organized a protest last week were rounded up.
Veteran dissident Hu Jia said authorities had stepped up security around him and other political activists in Beijing, according to Agence France Presse.
The strong action comes after Washington calling on Beijing to release those jailed since the Tiananmen protest, in which hundreds — and possibly thousands — of protesters were shot and killed by soldiers.
The State Department issued a statement saying it joined "the international community in remembering the tragic loss of innocent lives," CNN reported.
"We encourage the Chinese government to release all those still serving sentences for their participation in the demonstrations; to provide a full public accounting of those killed, detained or missing; and to end the continued harassment of demonstration participants and their families," the statement added.
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The call provoked an angry response from China, with Foreign Affairs Ministry spokesman Liu Weimin accused the US government of interfering with China's internal affairs, CNN reported.
"The US side has been ignoring the facts and issuing such statements year after year, making baseless accusations against the Chinese government and arbitrarily interfering with China's internal affairs," Liu reportedly said.
"The Chinese side expresses strong dissatisfaction and resolute opposition to such acts."
According to the BBC, terms including "Tiananmen," "square," "six four" and "never forget" and even "candle" return no information about the event when typed into Chinese search engines.
And officials had also targeted China's Twitter-esque microblogging site Sina Weibo, the Wall Street Journal reported, even blocking access to the number "23" as well as combinations of 4, 6, 8, and 9.
The Associated Press reported that due to the internet censors, the "Shanghai Composite Index" was blocked because the stock benchmark fell 64.89 points Monday, coincidentally matching the June 4, 1989 date of the Tiananmen protests.
Meanwhile, AFP quoted Zhou Jinxia from northeast China's Liaoning province as saying, "They brought in a lot of buses and were rounding up petitioners at the Beijing South rail station on Saturday night."
The 1989 pro-democracy protest, during which troops shot dead hundreds of demonstrators gathered in central Beijing — official Chinese government figures put the number of dead at 241, including soldiers — has never been publicly marked in China.
It was, however, marked in Hong Kong with a candlelight vigil, attended by 180,000 people. Lee Cheuk-yan, chairman of the Hong Kong Alliance in Support of Patriotic Democratic Movement in China, told the crowd, "Never forget June 4. Democracy for China now! Long live democracy," according to AFP.
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