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Users in Beijing, Shanghai and Shenzhen confirmed that since as early as Wednesday morning, Beijing time, they began receiving "unknown error" messages when they tried unsuccessfully to access the store. Although Apple does not operate an iTunes Store in China, users may download free content such as podcasts, and those with U.S. credit cards may buy content from the U.S. store.

Although Amazon.com remains available in China, its pages for both the "Songs for Tibet" CD and download page fail to load, returning: "The connection was reset. The connection to the server was reset while the page was loading," the most common error message received for blocked sites.

Attempts by IDG News Service to reach both the iTunes Music Store and the Amazon pages failed Friday morning, but succeeded when using a VPN (virtual private network).

The block comes during the final days of the Beijing Olympics, which has seen unfettered access to numerous politically-sensitive Web sites. It also comes almost exactly a month after the first Apple Store in China opened in Beijing, which signalled a more direct approach by the iPod maker to engage a market it previously saw mostly as a manufacturing base.

Apple did not respond to a request for comment.

David Wolf, head of Beijing-based consultancy Wolf Group Asia, who noted the blocking on his blog Wednesday morning, said "Apple has just learned that the choices you make across your business can affect your prospects in China. Success in China does not mean avoiding such conflicts, but in dealing with them intelligently and proactively." Wolf's blog is also blocked in China, as are all other Typepad-hosted pages.

"Songs for Tibet" includes music from artists including Sting, Alanis Morissette and John Mayer and is intended as "to support peace initiatives and Tibetan cultural preservation projects important to the Dalai Lama."

China sees Tibet as an inseparable part of its territory, whereas most Tibetans seek support independence for their homeland. The Chinese government regards the Dalai Lama as a "splittist," or separatist. The issue of Tibetan independence flared up in March, when anti-Chinese protests turned violent, resulting in the deaths of both Chinese and Tibetans. Pro-Tibetan independence activists overseas then targeted the Olympic torch relay with protests and attempts to seize the torch.