Webizens in China were frustrated Wednesday after the government blocked access to the social networking service Twitter and maintained its blackout on YouTube ahead of today’s 20th anniversary of the Tiananmen Square crackdown.

There are news blackouts on CNN and the BBC, and many blogs and newspaper sites had content censored ahead of the sensitive anniversary. There is no mention of the event in the Chinese-language media.

China has the world’s biggest Internet population, with 300 million users, and the government seems particularly worried about the speed at which information and pictures are spread by services such as Twitter and Flickr.

However, Facebook and MySpace escaped the Net nannies; and while Microsoft’s Windows Live is down, Hotmail, which was temporarily blocked, has been restored.

Search engines Google and Yahoo, which routinely censor search results such as “Tiananmen” and “Dalai Lama,” have not been affected.

Most Chinese dissidents from the 1989 movement are either in jail, in exile or, if still in China, have been forced to leave the city for the 20th anniversary.

China has never released a death toll from the crackdown on the student-led democracy movement, which the government classes as a “counter-revolutionary” act, insisting that the crackdown was necessary to pave the way for the country’s remarkable economic growth of recent years.

YouTube has been banned here for three months, probably because of video posted showing Tibetans being beaten up during last year’s riots.

This has led to some notable ironies. A few weeks ago, state broadcaster CCTV was raving about the YouTube Orchestra, where individuals play separately, via YouTube, a composition by Chinese musician Tan Dun.

However, the report failed to mention that locals will not be able to see the orchestra because of the ban.

China employs thousands of Net nannies to police the Internet. It has blocked YouTube before, but this blackout is the longest so far.