In late 2017 Britain declassified data its diplomats and intelligence agencies collected on what went on in June 1989 when the Chinese military savagely suppressed the main (and most reported on by world media) pro-democracy demonstration in China. All this took place in Beijing, the capital, in Tiananmen Square. The British collected numerous eye-witness accounts of the massacre itself, including the use of troops who were largely illiterate and known to be particularly loyal to the communist government. The Chinese government went to extreme lengths to suppress any data on how many actually died. The official number was 300 while Chinese Red Cross initially released estimates of 2,700 dead but later withdrew that. It turned out that during the massacre Chinese ambulances and other medical personnel were turned away and some of those who got through and did not leave quickly enough were massacred as well.
The British estimate agreed with the American intelligence estimate (over 10,000 dead and 40,000 wounded) that was released in 2014. The British data indicates the number of dead was somewhat higher and Chinese officials who spoke to British diplomatic personnel indicated that the government was not seeking an official number, even if highly classified but the number regularly used by Chinese officials was “at least 10,000.”) The Chinese sources are reliable but will never be named by the Brits, something they are good at keeping secret.
The British released a lot of details on how the attack was carried out, including the use of promises (quickly broken) to allow demonstrators (including survivors of the first round of killing) to leave. The troops apparently had orders to kill all civilians in the square and destroy the bodies where they fell. This included crushing the dead using armored vehicles, burning those remains and flushing those remains down storm drains. The area was sealed off for over a month so the cleanup could be thorough.
After the June 4 massacre the Chinese government enacted a growing number of measures to erase the June 1989 demonstrations (there were over 400 different demonstrations across the country with millions of Chinese participating) from popular memory. Martial law was imposed on the capital until 1990 to make it easier to hunt down survivors and terrorize anyone inclined to discuss the massacre.
The censorship effort continues. A good example occurred in 2015 when Hong Kong was the scene of over 100,000 people gathering to commemorate the 26th anniversary of the Tiananmen Square massacre. Hong Kong residents have more freedoms because of a 1990s deal with Britain to leave their prosperous colony intact. Thus Chinese in Hong Kong never forgot Tiananmen. This spontaneous uprising scared Chinese officials a great deal as they saw it as potentially a Chinese version of the 1989 collapse of communist rule in East Europe that occurred by the end of 1989. Every year at this time Chinese Internet censors are noticeably more active in a continuing effort to keep any news of the 1989 uprising from the Chinese public. Any discussion of the savage crackdown on pro-democracy demonstrators has been banned since 1989. The government effort has been successful at keeping most Chinese from knowing the details, or caring much about it. However many Chinese are aware that something happened. There are so many nasty aspects of Chinese history that Chinese are dimly aware of but not particularly curious about. In China there is a lot to forget and good reasons for doing so.