"YouTube has been blocked in China since yesterday," company spokesman Scott Rubin said.
"We do not know the reason for the blockage, and we're working as quickly as possible to restore access to our users in China."
Chinese authorities have a history of blocking websites they deem politically unacceptable or offensive.
A spokesman for the Chinese consulate in San Francisco told AFP he didn't have any information about YouTube being blocked in China.
The YouTube blockage came as government officials there publicly challenged the authenticity of a video that purports to show police beating to death a pro-Tibet demonstrator last year.
China's official Xinhua News Agency quoted an unidentified government source saying supporters of the Dalai Lama were "fabricating lies" by doctoring video to "deceive the international community."
In March last year access to the site was barred in China after video clips began appearing on the site showing violent unrest in the Tibetan capital Lhasa that triggered a virtual lockdown of the city by security forces.
Footage of Chinese troops apparently beating Tibetans last year in and near Lhasa following the deadly riots has appeared on YouTube in recent days. The source, date and location of the footage, posted by a Tibetan exile group, could not be independently confirmed.
The video shows hundreds of uniformed Chinese soldiers running through a Tibetan monastery, some of them beating a man with batons.
In another scene, uniformed soldiers kick, drag and beat several men and women who are lying on the ground, some of them with their hands bound behind their backs.
The narrator of the video said the violence was part of China's crackdown following anti-Chinese unrest in the Tibetan capital March 14, 2008, which Beijing says led to the deaths of 21 people by rioters.
Earlier this March China deployed a heavy security presence in Tibet and neighboring provinces to head off any unrest ahead of two significant anniversaries.
In addition to the March 14 anniversary, March 10 marked the 50th anniversary of a failed uprising against Chinese rule that led to Tibet's spiritual leader the Dalai Lama fleeing the Himalayan region for India.
Marc van der Chijs, a Dutch Internet entrepreneur who co-founded Shanghai-based video-sharing website Tudou.com, offered another theory for the YouTube blockage in a Tuesday message on his website.
"I suspect the real reason might be that YouTube just launched a Chinese version, which would make the site much more accessible for Chinese users," van der Chijs wrote.
"Not a very smart idea to do that in the middle of the National Congress, and I am surprised nobody at mother company Google's China offices rang an alarm bell about this before launch."
California-based Google bought YouTube in 2006 in a 1.65-billion-dollar stock deal.
"I don't like sites to be blocked; even not those of our competitors," van der Chijs wrote. "But, it will be an interesting discussion point for our Tudou board meeting tomorrow."
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