The popular Internet photo site Flickr said that its service is being blocked in China, although the Yahoo subsidiary did not directly blame the Beijing government, which aggressively censors the Internet of material it deems subversive.

The blocking, which began Thursday, is keeping Internet users across a large part of China from viewing photos on Flickr, home to millions of snapshots of everything from birthday parties to beach vacations to nudes.

The Web site also hosts a smattering of images that may be frowned upon by Chinese censors, including student protesters in Tiananmen Square in 1989, which includes the famous photo of a man blocking the progress of Chinese army tanks, and bodies of students who were killed in the streets as part of a government crackdown.

China's tight control over the Internet has become a high-profile issue in recent years as the online world makes increasing inroads with its vast population. Authorities routinely block access to online information about political opposition groups, Taiwanese independence and overseas Web sites such as BBC News, prompting outrage from human rights advocates.

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U.S. Internet companies have been caught in the middle, forced to weigh free expression against building their businesses in a potentially lucrative market. Mountain View's Google and Sunnyvale's Yahoo have faced intense criticism for their positions, which have included censoring search results and providing information about dissidents to police.

In postings on its Web site Thursday, Flickr said it was experiencing no technical problems and that its service was in fact being blocked, without saying by whom. In an update Friday, co-founder Stewart Butterfield wrote that the Web site's staff is checking on the issue periodically and that the blocking continues.

"We hope that this is a temporary issue, and we currently believe that it will be," he said in the posting. "In the meantime, we are investigating our alternatives."

Telephone and e-mail messages left with Butterfield and a Yahoo spokesman were not returned.

Users from across China posted messages on Flickr describing difficulties accessing images on the Web site, voicing frustration and laying blame on the Chinese government. Access to the Flickr home page and comments area is still apparently possible in China.

Jain Hua Li, a spokesman for the Chinese Embassy in Washington, said he hadn't heard of Flickr until told about it in a conversation with a Chronicle reporter, and then suggested that the blocking may be because Chinese authorities are trying to protect children from racy images.

Lucie Morillon, the U.S. representative for Reporters Without Borders, a French group that promotes free expression, said that the Beijing government often censors Web sites under the guise of protecting children or national security. She called the blocking of Flickr "one more blow against the free flow of information online by Chinese authorities" and added that it is particularly lamentable in light of promises by China to loosen restrictions before the 2008 Olympics in Beijing.