It's known abroad as "the Great Firewall of China": a website blacklist that limits what Chinese Internet users can see as they surf the Web.

Sometimes the reasons are obvious, such as when the firewall blocks websites promoting Tibet separatism or reporting oppression against Falun Gong practitioners. Sometimes they are less so. For much of the day Wednesday, Chinese Internet users were apparently blocked from access to Facebook and Fox News, according to a nonprofit site that monitors Chinese censorship.

No such blacklist operates in the United States - yet. But opponents of antipiracy legislation nearing a vote in Congress say the prospect isn't as far-fetched as it might sound - not because of the aims of those backing the bills, but because of their ham-handedness.

"Once you open up the door, who knows where it leads?" says Julie Samuels, a lawyer at the Electronic Frontier Foundation, a San Francisco nonprofit that helped organize a nationwide protest Wednesday against the House's Stop Online Piracy Act (SOPA) and the Senate's sister bill, the Protect Intellectual Property Act (PIPA).

To dramatize worries about the legislation, which is backed by the film and music industries, thousands of U.S. websites went dark for the day or steered visitors to information about the bills. As intended, many congressional offices reported being swamped with calls.

Wikipedia, the user-created online encyclopedia that has periodically run afoul of Chinese censors, blocked all its English-language pages with a warning headlined "Imagine a World Without Free Knowledge."

"For over a decade, we have spent millions of hours building the largest encyclopedia in human history," Wikipedia said. "Right now, the U.S. Congress is considering legislation that could fatally damage the free and open Internet."

The rhetoric may sound over-the-top - as do responses from backers of the antipiracy bills such as Rupert Murdoch, chief executive of News Corp., Fox News' owner, who lashed out at newfound White House wariness about the bills: "So Obama has thrown in his lot with Silicon Valley paymasters who threaten all software creators with piracy, plain thievery."

But the underlying issues - and the threat to the Internet from bad legislation - are hardly overblown.

"They use really wide-reaching tools to attempt to solve a problem that is unreachable by legislation," Samuels says. "And in doing so, they harm free speech, they harm innovation, and they harm the same democratic ideals that the Internet has been so successful at fostering."

As anyone who owns intellectual property will tell you, piracy is a real problem that has been magnified by computer technology.

Rip-offs of a Phillies jersey or a Rolex watch may fool the uninitiated, but they are at least identifiable by experts. A digital copy is a perfect copy. As a result, those who write books, record music, or create movies are vulnerable to a kind of theft that never existed before.

But Samuels says the legislation exaggerates the risks of copyright infringement while threatening major collateral damage. Under some of the more controversial provisions, for instance, Internet service providers could be forced to block certain URLs - in essence, creating blacklists like those used by China's censors.

"Some people are going to find a way to make copies whether or not SOPA or PIPA ever become law," Samuels says. "Our concern is that we're talking about taking down - blocking access to - full websites." And a site could be vulnerable simply for linking to another site accused of distributing pirated copies, she says.

Or perhaps even by mistake - a possibility Samuels says is illustrated by the saga of Dajaz1.com, a hip-hop music website whose Internet domain was among 82 seized in November 2010 by U.S. Immigration and Customs Enforcement because they were "engaged in the illegal sale and distribution of counterfeit goods and copyrighted works."

Although it was dark Wednesday to join in the campaign against the new antipiracy laws, Dajaz1.com is back online. According to TechDirt.com, which investigated the incident, the government has essentially "admitted that it totally screwed up and falsely seized and censored a non-infringing domain of a popular blog."

Samuels says TechDirt's investigation, which has prompted questions from Congress, showed that the allegedly pirated material came from representatives of the copyright-holders.

"The site was disappeared off the Internet for over a year for no good reason," Samuels says.

In other words, it could happen here - because it has happened. And if it does, we've all been warned.

Contact columnist Jeff Gelles

at 215-854-2776 or jgelles@phillynews.com.