Many are posting photos of candles online, while others are wearing black T-shirts, Voice of America reported. Tuesday.
Police began standing guard to ward off protests at the square on Friday where Chinese troops had crushed a student-led demonstration. Estimates of the number killed have ranged from several hundred to several thousand, but the government has never commented on the number of protesters who died.
Any mention of the event is blocked on the Internet and social media. A candle icon that served as a digital vigil was removed from Sina Weibo, the Chinese version of Twitter. In response, citizens posted photos of candles.
In Hong Kong, where residents enjoy greater freedom, more than 100,000 people were expected to attend a candlelight vigil.
Hu Jia, an AIDS activist and political dissident in Beijing, urged people attending the vigil to wear black, The Wall Street Journal reported on its blog.
He joked that buying black T-shirts in China might soon require identification.
Government censorship of the Internet and social media has made commenting directly on the anniversary difficult. Some Sina Weibo users instead noted the arrival of dark clouds in Beijing around midday Tuesday.
"Heaven sees what the people see, Heaven hears what the people hear. Today, Heaven's heart feels what's in the people's hearts," wrote historian Zhang Lifan, quoting Confucius.