Twitterers around China are reporting that the popular micro-blogging site Twitter.com appears to be blocked, the first time the site has been widely inaccessible to users in the country.

Twitter users began reporting difficulty getting on the site late on Tuesday afternoon in China, just days ahead of the sensitive 20th anniversary of the Tiananmen Square crackdown. Twitterers said access was still possible through some Twitter clients, such as Tweetdeck, but users trying to access their accounts through other clients, such as Twhirl, said they also encountered problems.

In the past, Twitter has proved remarkably free of interference. It gained prominence in China during the immediate aftermath of last year’s big earthquake in Sichuan.

But in recent weeks, activist groups have reported a tightening of security in China, linking it to the upcoming Tiananmen anniversary. Human Rights in China, based in New York, said Tuesday that authorities have detained an elderly freelance writer from Taizhou in Zhejiang province, who published 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). The controls also appear to be spreading to the Internet, though it’s always difficult to tell whether a site has been purposely blocked or whether other technical problems may be to blame.

The Chinese government doesn’t comment on specific access issues, but sites such as YouTube and Flickr are intermittently unavailable to users in China, especially around important government meetings or anniversaries. YouTube has been blocked for several weeks in China, according to reports on Harvard University’s Herdict Web. Users in China also reported that Flickr was inaccessible on Tuesday afternoon.

True to form, Twitterers quickly adopted a hash tag for Tweets on the service’s problems: #twitterblock. Users also pushed #gfw, which stands for “great firewall,” into the top trending topics on Tuesday.

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