When Shanghai blogger Isaac Mao tried to watch a YouTube clip of Chinese police beating Tibetans, all he got was an error message.

Mao thought the error — just after the one-year anniversary of a crackdown on Tibetan protesters in China — was too suspicious to be coincidental, so he reported it on a new Harvard-based Web site that tracks online censorship.

Meanwhile, more than 100 other people in China did the same thing. The spike in reports on Herdict.org in March pointed to government interference rather than a run-of-the-mill technical glitch, even before Google Inc. confirmed China was blocking its YouTube video-sharing site.

"We saw reports coming in as soon as the blocks were happening and certainly before any of the media were reporting it," Herdict founder Jonathan Zittrain said of the months-long YouTube blackout that coincided with the 20th anniversary of the Tiananmen Square demonstrations in June and recent ethnic riots in the Xinjiang province.

Herdict users report their Web site problems anonymously — numeric Internet addresses are recorded but only general location is displayed — so people can post more freely, encouraging reports about sensitive topics like HIV and AIDS-related sites, and from people in countries with possible government repercussions.

The site doesn't investigate reports, though, so there's no way to know for sure that an outage is related to government meddling rather than a cut cable or other problem unrelated to censorship. Although surges in reports do suggest a government role, a widespread technical glitch can also produce a similar spike.

Web site inaccessibility can also result from network or server errors, firewalls at schools or offices or a new phenomenon called reverse filtering, in which companies block access to copyright-protected material outside a specific country.

Zittrain, law professor and co-founder of Harvard's Berkman Center for Internet and Society, said Herdict does not aim to present a flawless picture of online filtering, but to let patterns of accessibility speak for themselves.

"The goal ... is to gather the kind of raw data from which people can then start to gain insight and come to conclusions," he said. "With enough people asking, you start to get a sense of where there are blockages in the network."

Herdict — short for "verdict of the herd" — has spread beyond techie circles to garner users in more than 140 countries, including censorship hotbeds China and Iran.

"Herdict has been buzzed (about) for months in China and now it's becoming more popular since ... Google.com was blocked for hours and Twitter.com was blocked twice recently," Mao said in an e-mail.

In Iran, Herdict users have logged unsuccessful attempts to access Twitter and other social-networking sites that have been blocked since the country's controversial June 12 presidential election.

Herdict users like that the site fosters a sense of community among those who can't fully navigate the Web and provides them with hope for a freer Internet.

"It gives people a sense how many people share the same blackout regionally or globally," Mao said. "You are not alone."

Before, someone might complain about a block via a single Facebook or Twitter update, but that information often doesn't go beyond a small group of friends.

Zittrain started Herdict in February — a month before China's block began — to aggregate reports of online inaccessibility and help users detect government censorship on the Web as soon as it happens. Having tracked online censorship since the early 2000s, he wanted to put Web accessibility at the fingertips of those who use it most, rather than a handful of experts.

"The less 'online' class of people generally don't worry about it, until they run into something blocked like the BBC," said Andrew Lewman, executive director of the Boston-based circumvention tool, The Tor Project Inc. "Then they say, 'Hey, what is this? All I want to do is read this one article.'"

The site has versions in Arabic and Chinese, and an interactive map with a roaming orange sheep to mark inaccessible Web sites.

Don't expect censorship to go away, though. At most, Herdict can help give people a better sense of the prevalence of censorship.

"I don't think that a specific monitoring tool will specifically have censorship go away, but we'll just know about it better," said Robert Guerra, project director for the Internet Freedom Program at the Washington-based Freedom House. "It's far more pervasive than people think."