Chinese media site Danwei says Twitter users across China are reporting that their accounts are being blocked by government censors.
The move comes in the lead up to Thursday’s anniversary of the Tiananmen Square massacre.
While its usual for the Chinese government to step up media censorship ahead of any big event, commentators says this is the first time that Twitter and other social media sites have been explicitly targeted.
Other apparently blocked sites include the Yahoo-owned photo-sharing site Flickr, and Microsoft’s newly-unveiled search engine, Bing.
Ironically, Bing seems to be blocked because of a feature that is also drawing flak closer to home: the fact it plays full videos (including those of Google YouTube clips) in its search result thumbnail previews. To the Chinese, it's subversive, to some in the west, it's a copyright threat.