China's censors blocked internet access to the terms "six four", "23", "candle" and "never forget" on Monday, broadening extensive efforts to silence talk about the 23rd anniversary of China's bloody June 4 crackdown on pro-democracy protesters.
For China's ruling Communist Party, the 1989 demonstrations that clogged Tiananmen Square in Beijing and spread to other cities remains taboo, all the more so this year as the government prepares for a tricky leadership handover.
Searches for the terms related to the anniversary, such as "six four" for June 4, were 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," a Sina microblogger wrote. Sina was not immediately available for comment.
The anniversary of the date on which troops shot their way into central Beijing in 1989 has never been publicly marked in mainland China.
The government has never released a death toll of the crackdown, but estimates from human rights groups and witnesses range from several hundred to several thousand.
U.S. State Department deputy spokesman Mark Toner urged the Chinese government on Sunday "to provide a full public accounting of those killed, detained or missing".
"tragedy that should have been averted"
Microbloggers decried the overzealous rash of censorship, complaining that their posts had been "harmonised" - a euphemism for censorship - within minutes.
Censors also prevented microbloggers from changing their display photos in an apparent attempt to prevent them from posting any photo commemorating the anniversary.
Yet some people did manage to beat the censors, and a few pictures of the 1989 protests did find their way on to Weibo.
"There can be no social stability if people cannot speak out and must live in terror of punishment," a microblogger commented on one of the photographs.
Yao Jianfu, author of a new book of interviews with Chen Xitong, the Beijing mayor at the time of the crackdown, told Reuters that Chen had said "this was a tragedy that should have been averted but wasn't".
"I never foresaw there would be shooting, because Mao Zedong said that ordinary people should not be shot at and suppressing student protests comes to no good," said Yao.
The government has restricted the movements of dozens of dissidents, former prisoners and petitioners during the anniversary period and warned them against speaking to journalists or organising activities, said Songlian Wang of rights group Chinese Human Rights Defenders.
A coalition of lawyers and rights activists began a one-day fast in their homes on Monday to commemorate the anniversary, said a Shandong-based lawyer, Liu Weiguo.
Tens of thousands of people are expected to attend a candlelight vigil in Hong Kong, said organisers, who had erected a replica of the Goddess of Democracy that was built in Tiananmen Square in 1989.
Chinese tourists stopped on Tiananmen Square shook their heads and appeared mystified when asked about the anniversary. There were no obvious signs of extra security on the already well-guarded square.
But a trinket vendor said he was well aware what day it was.
"Do foreigners also know about June 4?" he asked a Reuters reporter in a hushed tone, looking around to make sure nobody heard him. "I think it is important we remember but nobody will talk about it now."