Tens of thousands of people attend a candlelight vigil at Victoria Park in Hong Kong, June 4, 2014, to mark the 25th anniversary of the June 4 Chinese military crackdown on the pro-democracy movement in Beijing. (Kin Cheung/AP)

Tens of thousands gathered in central Hong Kong today to mark the 25th anniversary of the Tiananmen Square massacre, even as mainland China tightened security to forestall protests.

The student movement in 1989, which briefly stoked hopes of democratic reforms in the world's most populous nation, ended in a brutal crackdown that killed hundreds of civilians, many of whom students.

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In addition to increased security presence, the Chinese government has blocked almost all Google services, including Gmail, searches, translation and images since last week, watchdog groups noted.

Since 1990, Hong Kong has been marking the crackdown with a candlelight vigil in Victoria Park on June 4 every year.