Tens of thousands of people turned out for a candlelit vigil in Hong Kong in memory of China's Tiananmen Square massacre.
A bloody crackdown on pro-democracy protesters in and near Beijing's Tiananmen Square in 1989 left an unknown number of people dead and injured.
China has never released a death toll, but estimates by human rights groups and witnesses range from several hundred to several thousand.
China moved to block internet searches and online references to the massacre.
The Hong Kong demonstrators held candles around a memorial and a replica of the Goddess of Democracy statue that was built in Tiananmen Square before tanks and troops crushed the protests.
The anniversary has never been publicly marked in mainland China.
Instead, China's censors blocked access to the term "Shanghai stock market" on popular microblogs after the index fell a bizarre 64.89 points, matching the date of the crackdown.
In another twist, the Shanghai Composite Index opened at 2346.98 points on the 23rd anniversary of the killings. The numbers 46.98 are 4 June 1989, backwards.
"Whoa, these figures are too freaky! Very cool!" said a microblogger. "The opening figure and the drop are both too creepy," said another.
The Shanghai Stock Exchange said it was investigating.
For China's ruling Communist Party, all discussion of the 1989 demonstrations that clogged Tiananmen Square and spread to other cities remains taboo, all the more so this year as the government prepares for a tricky leadership handover.
But for Hong Kong, a former British colony which enjoys wide-ranging autonomy, the 4 June vigil is an annual event.
Terms related to the anniversary, such as "six four" for4 June, were also blocked on Sina Weibo, the most popular of China's Twitter-like microblogging platforms.
Users encountered a message that said the search results could not be displayed "due to relevant laws, regulations and policies".
"It's that day again and once more numerous posts are being deleted," wrote a microblogger.
US State Department deputy spokesman Mark Toner urged the Chinese government "to provide a full public accounting of those killed, detained or missing".
Chinese Foreign Ministry spokesman Liu Weimin said the US habit of issuing a statement on each anniversary amounted to "crude meddling in domestic Chinese affairs".
While state media made no direct mention of the anniversary, the top military newspaper ordered the armed forces to be on guard for attempts to sow unrest ahead of the party congress which will handle the change of leadership.
The government is grappling with the biggest political scandal since the 1989 crackdown - the crisis that followed Chongqing police chief Wang Lijun's 6 February flight to the US consulate in Chengdu and toppled Chongqing party chief Bo Xilai.