By Maggie Shiels
Technology reporter, BBC News, Silicon Valley
YouTube is the site that is most often banned or blocked
China's move to block YouTube has been criticised by a leading advocacy group that promotes constitutional liberties in the digital age.
The Centre for Democracy and Technology told the BBC: "China's actions fail to live up to international norms."
The video sharing site has been off limits in China since Monday.
"China's apparent blocking of YouTube is at odds with the rule of law and the right to freedom of expression," said CDT president Leslie Harris.
"Anytime a country limits or takes down content online , it must be forthright and specific about its actions and do so only in narrowly defined circumstances consistent with international human rights and the rule of law," stated Ms Harris.
Google, which owns YouTube, told the BBC that it had no idea why the Chinese government had taken this action.
"We don't know the reason for the block and are working to restore access to users in China as quickly as possible," said spokesperson Scott Rubin.
Earlier in the week, the BBC reported from Beijing that China cut off access to the website because it carried a video showing soldiers beating monks and other Tibetans.
The graphic video was released by Tibetan exiles and showed hundreds of uniformed Chinese troops swarming through a Tibetan monastery. It included footage of a group of troops beating a man with batons.