By JOSH CHIN

BEIJING—China's Internet monitors have unleashed a broad clampdown on online discussion of the 23rd anniversary of the Tiananmen Square crackdown, restricting even discussion of the nation's main stock market when it fell by a number that hinted at the sensitive date.
Officials minding China's popular Twitter-like microblogging service Sina Weibo beginning this weekend began blocking a number of terms that could refer to the 1989 Tiananmen Square crackdown, an incident often referred to as June 4 or 64 in the Chinese-speaking world. Under the crackdown the government ordered troops to fire on unarmed demonstrators, likely killings hundreds.
Terms blocked by Sina Weibo included the Chinese characters for "Tiananmen," "square," and "candle," and even seemingly innocuous words like "today." It also included numbers that could allude to the event, including the number "23" as well as combinations of 4, 6, 8, and 9.


Agence France-Presse/Getty ImagesChinese stock investors monitor their share prices in Huaibei, east China's Anhui province, on Monday. The Shanghai Composite closed down 64.89 on Monday, evoking the date of the Tiananmen Square crackdown on June 4, 1989, and prompting China's Web monitors to block discussion of the stock market close online.

The clampdown spread discussion in the business and financial world—often a more open topic of conversation in China's online and traditional media—when the benchmark Shanghai Composite Index ended the day down 64.89 points. Sina Weibo then blocked use of the terms "index" and "Shanghai Index."
A spokesman for Sina Weibo operator Sina Corp. didn't immediately respond to a request to comment. Because Chinese officials often pressure companies themselves to strictly regulate content, it was unclear whether the tighter reins of recent days were ordered up by Beijing.
A media representative at the Shanghai Stock Exchange said trading opened normally on Monday. He didn't immediately answer additional questions.
At a daily media briefing, China Foreign Ministry spokesman Liu Weimin said the nation had reached a "clear conclusion" over the Tiananmen Square crackdown, adding that since then China has enjoyed "continuous economic development, democracy [and] rule of law," adding, "all of this reflects the common aspiration of the Chinese people."
Mr. Liu also rejected a Sunday statement from the U.S. State Department calling on China to respect human rights and to end harassment of participants and their families. "It is a gross intervention into China's internal affairs and a groundless accusation [against] the Chinese government. China expresses its strong dissatisfaction and firm opposition to that."
The Tiananmen Square anniversary has pushed the typical cat-and-mouse game between Internet and censors to new heights at a time when online companies are coming under increasing pressure to rein in politically sensitive discussion on the site. China is approaching a once-a-decade leadership change that begins later this year, and disturbances ranging from the ouster of former Communist Party star Bo Xilai and a slowing economy threaten to complicate Beijing's hopes for a smooth transition.
Both Sina and rival Tencent Holdings Ltd. have been attempting to meet a government demand that all microblog users register with their real names, which analysts say is part of an effort to improve monitoring of online speech. Sina has also recently introduced a points-based system for measuring user behavior Weibo, widely described as a warning to users to be careful about what they post.
"There's no question that the censorship is really being ratcheted up on social media," said David Bandurksi, a researcher with Hong Kong University's China Media Project, adding that China's propaganda authorities have taken a particular interest in Sina Weibo.
"There are these groups of people who can use Sina Weibo to sort of wink at each other say 'We know what this day is," he said. "You have a dispersed online population that share with each other through these hints and suggestions."
The Sina Weibo clampdown claimed a number of seemingly anodyne terms and images, including the emoticon of a candle that is usually available to users. It also claimed "Victoria Park," the site in Hong Kong of an annual memorial to the event. It also claimed the number 35 because users hoping to fool censors sometimes refer to the June 4 event as May 35.
"Say nothing," one user wrote. "Everyone understands."
Users still found ways to discuss the event. Some began quoting a poem by ninth century literary figure Li Shangyin that includes the lines "A spring silkworm spins its thread until its dying day / A candle's tears do not dry until it has burned down to ash." The line is an apparent reference to the disappearance of the candle icon as well as the blocking of the term "candle," though the poem uses an older character for that word that wasn't yet blocked Monday afternoon.
"The candle has burned down to ash, but its tears have still not dried," wrote one user posting under the handle Futures Gambler, attaching a photo of candles arranged to spell out "6.4."
Another user posted a large photo of a single candle flame with the message "You understand."
"I understand," another user wrote in the comments under the post. "My face is covered in tears."
—Andrew Galbraith in Shanghai and Carlos Tejada in Beijing contributed to this article.Write to Josh Chin at [email protected]