Twitter Inc. users across China reported that the popular networking service appeared to be blocked Tuesday, two days ahead of the sensitive 20th anniversary of the military crackdown on pro-democracy protesters at Tiananmen Square.
If the site is being blocked by government censors, as many users suspect, it would mark the first time that Twitter has been widely inaccessible to users in China.
The so-called microblogging service, which garnered attention domestically during the immediate aftermath of last year's earthquake in Sichuan, hasn't previously been subject to restrictions in China. As a result, a number of prominent Chinese activists use Twitter regularly, either under their own names or using aliases.
Still, it's often difficult to tell whether a Web site has been purposely blocked by Chinese authorities, if other technical problems are to blame, or if services are blocked only in certain areas.
Officials at the State Administration of Radio, Film and Television, China's main media regulator, couldn't be reached late Tuesday, when word of the outages first spread. Chinese regulators in the past have declined to comment on potential stoppages.
A Twitter spokeswoman didn't have an immediate comment and couldn't confirm whether the service was blocked in China.
Sites that include large amounts of user-generated content are intermittently unavailable to users in China, especially around important government meetings or anniversaries. YouTube, Google Inc.'s video-sharing site, has been blocked for several weeks in China and remains so, a Google spokesman said Tuesday.
Some users said it was still possible to use Twitter through certain software applications, such as Seesmic, that allow users to send and receive messages without directly using the Twitter.com Web site. But users trying to access their Twitter accounts through other programs said they encountered problems. Fanfou.com, a popular domestic site that is similar to Twitter, appeared to be functioning normally on Tuesday.
Twitter has given the Internet-savvy in China another new platform to voice complaints and race ahead of state-controlled news media platforms. The development of Internet channels has pressured the government to respond to news faster.
Also Tuesday, a magazine published by the official Xinhua news service called on local governments to respond more quickly and develop a greater online presence to respond to popular online movements.
The Chinese government considers the 1989 pro-democracy protests to have been a counterrevolutionary riot, and further discussion of them remains taboo. The police presence around Tiananmen Square has increased, and on Tuesday the Foreign Correspondents Club of China issued a statement protesting restrictions on journalists attempting to cover the anniversary.
In recent weeks, activist groups have reported a tightening of security in China, linking it to the coming anniversary. Human Rights in China, based in New York, said that authorities detained Wu Gaoxing, a free-lance writer from Taizhou in Zhejiang province, who co-wrote an open letter to China's top leaders recently asking for equal rights and social security for ex-Tiananmen Square prisoners. China's Ministry of Public Security deferred requests for comment to Taizhou's local public security bureau, which declined to answer any questions.
Word of Twitter's outage became a popular subject on the service's site Tuesday. A number of users adopted an obscene variant of a hash tag derived from the acronym for the Great Firewall, the nickname for China's Internet censorship efforts. Hash tags are used on Twitter to mark and link to similar posts.
—Loretta Chao in Beijing contributed to this article.