BEIJING  China’s government censors have begun to block access to the Internet services Twitter, Flickr, Hotmail and Microsoft’s live.com, broadening an already extraordinary effort to shield its citizens from any hint of Thursday’s 20th anniversary of the military crackdown that ended the 1989 Tiananmen Square pro-democracy movement.

People in China who tried to gain access to the blocked Web sites on Tuesday instead encountered an error message saying the sites’ servers had unexpectedly dropped the Internet connection  a standard indicator that access has been blocked.

Weeks earlier, censors blocked Chinese users from viewing all videos on YouTube, and in recent days some television viewers have reported that BBC World News reports related to the Tiananmen anniversary were being selectively blacked out of broadcast programs.

Government censorship of political material on Internet bulletin boards and Web sites is common in China, but this is the first time Twitter has been blocked. Some well-known political activists, unable to post comments on Chinese blogs or chat sites, had switched to Twitter in recent months as an uncensored outlet for their views.