The crackdown on pro-democracy protesters on June 4, 1989, killed hundreds, possibly more. The Chinese government has never fully disclosed what happened on that day and branded the protests a "counterrevolutionary riot".
It remains a taboo topic inside the country, but the growing use of Twitter-like sites known as Weibo and other social media -- although largely censored -- has made it difficult for authorities to control all information about the Tiananmen crackdown.
"Thanks to the role of Weibo, there are now more people than any other time in the past 24 years that have come to know and think about the incident," he said.
While protests in mainland China are off-limits, tens of thousands have gathered to mark the anniversary in past years in Hong Kong's Victoria Park. The territory, returned by the British in 1997, operates under a separate political system that promises freedom of speech and other Western-style civil liberties.
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Because of restrictions placed on him as an activist, Hu will be unable to leave his home to mark the anniversary. He said controls placed on him for this year's event, the first since Xi Jinping became leader, were tighter than before.
"Last year, I was restricted from going out for shopping, visiting my parents and friends during the period between June 2 and June 5, and this year the restrictions were imposed on me as early as May 25," he said.