NEW YORK (AP) — Can the world live without Wikipedia for a day? The shutdown of one of the Internet's most-visited sites is not sitting well with some of its volunteer editors, who say the protest of anti-piracy legislation could threaten the credibility of their work.

"My main concern is that it puts the organization in the role of advocacy, and that's a slippery slope," said editor Robert Lawton, a Michigan computer consultant who would prefer that the encyclopedia stick to being a neutral repository of knowledge. "Before we know it, we're blacked out because we want to save the whales."

Wikipedia's English-language site shut down at midnight Eastern Standard Time Tuesday and the organization said it would stay down for 24 hours.

Instead of encyclopedia articles, visitors to the site saw a stark black-and-white page with the message: "Imagine a world without free knowledge." It carried a link to information about the two congressional bills and details about how to reach lawmakers.

It is the first time the English site has been blacked out. Wikipedia's Italian site came down once briefly in protest to an Internet censorship bill put forward by the Berlusconi government. The bill did not advance.

The shutdown adds to a growing body of critics who are speaking out against the legislation. But some editors are so uneasy with the move that they have blacked out their own user profile pages or resigned their administrative rights on the site to protest. Some likened the site's decision to fighting censorship with censorship.

One of the site's own "five pillars" of conduct says that Wikipedia "is written from a neutral point of view." The site strives to "avoid advocacy, and we characterize information and issues rather than debate them."

Wikipedia founder Jimmy Wales argues that the site can maintain neutrality in content even as it takes public positions on issues.

"The encyclopedia will always be neutral. The community need not be, not when the encyclopedia is threatened," he tweeted.

The Wikimedia Foundation, which administers the site, announced the blackout late Monday, after polling its community of volunteer contributors and editors and getting responses from 1,800 of them. The protest is aimed at the Stop Online Piracy Act in the House and the Protect Intellectual Property Act under consideration in the Senate.

"If passed, this legislation will harm the free and open Internet and bring about new tools for censorship of international websites inside the United States," the foundation said.

Both bills are designed to crack down on sales of pirated American products overseas, and they have the support of the film and music industry. Among the opponents are many Internet companies such as Google, Facebook, Yahoo, Twitter, eBay and AOL. They say the bills would hurt the industry and infringe on free-speech rights.

Social news website Reddit.com is shutting down for 12 hours on Wednesday, but most companies are staying up. Google Inc. said it will display its opposition to the bill on its home page in some fashion.

Dick Costollo, CEO of Twitter, said he opposes the legislation as well, but shutting down the service was out of the question.

"Closing a global business in reaction to single-issue national politics is foolish," Costollo tweeted.

Since Wikimedia depends on a small army of volunteers who create and update articles, it's particularly concerned about a lack of exemptions in the bills for sites where users might contribute copyrighted content. Today, it has no obligation under U.S. law except removing that content if a copyright holder complains. But under the House version of the bill, it could be shut down unless it polices its own pages.

The plans for the protest were moving forward even though the bill's prospects appeared to be dimming. On Saturday, Rep. Darrell Issa, a California Republican, said the bill would not move to the House floor for a vote unless consensus is reached. However, Lamar Smith, a Texas Republican, said work on the bill would resume next month.

The White House raised concerns over the weekend, pledging to work with Congress to battle piracy and counterfeiting while defending free expression, privacy and innovation in the Internet. The administration signaled it might use its veto power, if necessary.

That the bill seems unlikely to pass is another reason Lawton opposes the blackout.

"I think there are far more important things for the organization to focus aside from legislation that isn't likely to pass anyway," he said. He's been contributing to Wikipedia for eight years.

Danny Chia, another contributor to the site, said he had mixed feelings about the blackout. The neutrality applies to the content, but a lot of people interpret it as being about the site as a whole, said the Los Altos, Calif., software engineer.

In an online discussion, others raised the same point about the blackout: Appearances matter, and if the audience sees Wikipedia taking a stand, it might not believe the articles are objective, either.

Wikipedia has seen a small decline in participation, from a peak of 100,000 active editors a year ago to about 90,000 now. Wikimedia Foundation blames this mainly on outdated editing tools, and believes it can get the number growing again with software upgrades.