China appeared to block Twitter on Tuesday, following rising popularity for the service in China and just two days before the date when Beijing crushed pro-democracy protests in 1989.
The Twitter Web site would not load in China Tuesday evening, according to people contacted in the country.
Twitter users began posting messages about problems accessing the site or its application programming interface (API) address in the late afternoon. Applications that let users post Twitter messages without visiting the Web site require access to its API.
Messages on Twitter and one well-known blog also said the picture sharing site Flickr.com, Hotmail and new Microsoft search engine Bing became inaccessible Tuesday.
Some Chinese dissidents had appeared in recent months on Twitter, which provided a platform for free speech compared to the heavily patrolled blog services in China.
Twenty years ago on Thursday, hundreds died after Beijing ordered soldiers into its central Tiananmen Square to suppress peaceful demonstrations for democracy. China has stepped up monitoring of known dissidents ahead of the anniversary to prevent any public disturbances.