Google said Tuesday that its YouTube video-sharing Web site had been blocked in China.

Google said it did not know why the site had been blocked, but a report by the official Xinhua news agency of China on Tuesday said that supporters of the Dalai Lama had fabricated a video that appeared to show Chinese police officers brutally beating Tibetans after riots last year in Lhasa, the Tibetan capital.

Xinhua did not identify the video, but based on the description it appears to match a video available on YouTube that was recently released by the Tibetan government in exile.

It purports to show police officers storming a monastery after riots in Lhasa last March, kicking and beating protesters. It includes other instances of brutality and graphic images of a protester’s wounds. According to the video, the protester later died.

“We don’t know the reason for the block,” a Google spokesman, Scott Rubin, said. “Our government relations people are trying to resolve it.”

Mr. Rubin said that the company first noticed traffic from China had decreased sharply late Monday. By early Tuesday, he said, it had dropped to nearly zero.

China routinely filters Internet content and blocks material that is critical of its policies. It also frequently blocks individual videos on YouTube. YouTube was not blocked Tuesday or Wednesday in Hong Kong, the largely autonomous region of China. Beijing has not interfered with Internet sites there.

“The instant speculation is that YouTube is being blocked because the Tibetan government in exile released a particular video,” said Xiao Qiang, adjunct professor of journalism at the University of California, Berkeley, and editor of China Digital Times, a news Web site that chronicles political and economic changes in China.

Mr. Xiao said that the blocking of YouTube fit with what appeared to be an effort by China to step up its censorship of the Internet in recent months. Mr. Xiao said he was not surprised that YouTube was a target. It also hosts videos about the Tiananmen Square protests and many other subjects that Chinese authorities find objectionable.

The video about the beatings was pieced together from different places, Xinhua said on Tuesday, citing an unidentified official with the Tibetan regional government in China.

There has been no independent assessment of whether the video is authentic. In a statement sent via e-mail, Lobsang Nyandak, a representative of the Tibetan government in exile, said that the video was authentic.

The government did not directly address whether YouTube had been blocked. When asked about the matter at a news conference, a Foreign Ministry spokesman, Qin Gang, said: “Many people have a false impression that the Chinese government fears the Internet. In fact, it is just the opposite.”

Even as China steps up its censorship efforts, the country’s Internet participation is booming. Often, critics often find a way to avoid censors and debate controversial topics.

Ai Weiwei, a prominent Chinese artist, has been using his blog on Sina.com to criticize the government’s management of the rescue and relief efforts after the devastating earthquake in May in Sichuan Province.

In recent months, Beijing has announced major crackdowns on pornographic Web sites, even citing Google and other large companies for listing the sites on their search engines. Many critics say they believe that Beijing is using the word “pornography” as a rationale to eliminate Web sites that it deems troublesome.

YouTube has been blocked for varying periods of time in several countries, including Pakistan, Thailand and Turkey. These countries often state directly why they have acted.