The Chinese government has not officially commented on the move, but internet users in China trying to access YouTube get a blank screen. Other sites, such as the web pages for the BBC, The Guardian and Flickr, have also been blocked.
There have also been reports that internet cafes in Tibet are being shut down by the army, eager to avert a repeat of the footage that leaked out of Burmese protests against the ruling junta in that country.
"In terms of the media there is no question that the Chinese Government has imposed a lockdown of what they consider sensitive information or video footage, " Ying Chan, journalism and media studies director at the University of Hong Kong, told Al Jazeera.
"We are relying on tourists, citizens sending information out."
Video sharing sites based in China such as Tudou.com and 56.com are not hosting content about the Tibet unrest, which has already claimed many lives and appears to be spreading.
At the start of the year the Chinese government announced that video sharing sites would have to be state controlled but backed down after a week of heavy lobbying and said that the new laws did not apply to existing web sites.
China blocks YouTube over Tibetan videos
By Iain Thomson on Mar 18, 2008 7:21AM