"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."
The blockage comes as government officials in China labelled as "fake" a video posted on Youtube allegedly showing police beating a Tibetan demonstrator to death.
The official Xinhua news agency, citing an unidentified official with China's Tibetan regional government, said that the video came from sources tied to the government-in-exile of the Dalai Lama, the Tibetan spiritual leader, and was pieced together from different places.
Xinhua said the footage claimed to show an incident during last year's violent protests in Tibet against Chinese rule, but that the situation had been faked.
|The Chinese government says it is 'unafraid' of the internet [GALLO/GETTY]
"The Dalai Lama group is used to fabricating lies to deceive the international community and the aim of this video is to hide the truth of the March 14th riot," Xinhua quoted the official as saying.
Footage of Chinese troops apparently beating Tibetans during last years protests in and near the capital Lhasa has appeared on Youtube in recent days.
However, the source, date and location of the footage, posted by a Tibetan exile group, could not be independently confirmed.
One video shows uniformed Chinese soldiers running through a Tibetan monastery, some of them beating a man with batons.
In another scene, 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.
Last year's protests were the biggest display of anger at Chinese rule in many years.
In an effort to prevent a repeat of the unrest this year China deployed a massive security crackdown in Tibet and Tibetan areas, putting hundreds of police on streets and barring foreigners from the region.
China 'not afraid'
Following the apparent block on Youtube, China's foreign ministry said it was not aware of any restrictions on the site and the Chinese consulate in San Francisco told AFP it did not have any information on the issue.
Speaking to reporters in Beijing, Qin Gang, a ministry spokesman, rejected suggestions that the government was afraid of the internet.
"Many people have a false impression that the Chinese government fears the internet. In fact it is just the opposite," Qin said.
He said China's 300 million internet users and 100 million blogs showed that "China's internet is open enough, but also needs to be regulated by law in order to prevent the spread of harmful information and for national security".
Chinese authorities keep a tight rein on the internet and frequently bock access to websites the government finds objectionable for political or moral reasons.