China has blocked access to the term Shanghai Composite Index on some of the country's most popular microblogging sites.
This was after the index dropped by 64.89 points on Monday.
The numbers correspond to 4 June, 1989, the date of the crackdown against protesters at Tiananmen Square in Beijing.
China strictly prohibits references to the crackdown and has also censored other terms relating to the unrest.
