Wikipedia blacked out the English language version of its website today in protest at anti-piracy laws being considered by the US government.
Users attemping to access the site were met with a black screen and the statement: "Imagine a world without free knowledge."
The website, which shut down at 5am British time, will go dark for 24 hours, the foundation behind the popular community-based online encyclopaedia said, in an unprecedented move that brings added muscle to a growing base of critics of the legislation.
Wikipedia is considered one of the internet's most popular websites, with millions of visitors daily.
"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 Wikimedia foundation said.
The Stop Online Piracy Act in the US House of Representatives and the Protect Intellectual Property Act under consideration in the Senate are designed to crack down on sales of pirated US products overseas.
Supporters include the film and music industry, which often sees its products sold illegally. They say the legislation is needed to protect intellectual property and jobs.
Critics say the laws could hurt the technology industry and infringe free-speech rights. Among their concerns are provisions that would weaken cyber-security for companies and hinder domain access rights.
The most controversial provision is in the House bill, which would have enabled authorities to "blacklist" sites that are alleged to distribute pirated content. That would essentially cut off portions of the internet to all US users. But congressional leaders appear to be backing off from this provision.
Tech companies such as Google, Facebook, Yahoo, Twitter, eBay, AOL and others have spoken out against the legislation and say it threatens the industry's livelihood.
Several online communities such as Reddit, Boing Boing and others have announced plans to go dark in protest as well.
The Obama administration also raised concerns about the legislation over the weekend and said it would work with Congress on laws to help battle piracy and counterfeiting while defending free expression, privacy, security and innovation on the internet.
Wikipedia's decision to go dark brings the issue into a much brighter spotlight. A group of Wikipedia users have discussed for more than a month whether it should react to the legislation.
Over the past few days, a group of more than 1,800 volunteers who work on the site and other users considered several forms of online protest, including banner ads and a global blackout of the site, the foundation said. Ultimately, the group supported the decision to black out the English version of the site.
Jimmy Wales, the founder of Wikipedia who first announced the move on his Twitter account on Monday, said the bills were a threat to the free, open, and secure web.
"The whole thing is just a poorly designed mess," he said.
Wikipedia is also requesting that readers contact members of Congress about the bill during the blackout.
"I am personally asking everyone who cares about freedom and openness on the internet to contact their senators and representative," Mr Wales said.
"One of the things we have learned recently during the Arab Spring events is that the internet is a powerfully effective tool for the public to organise and have their voices heard."
This is the first time Wikipedia's English version has gone dark. Its Italian site came down once briefly in protest to an internet censorship bill put forward by the Berlusconi government; the bill did not advance.
"Wikipedia is about being open," said Jay Walsh, spokesman for the Wikimedia foundation. "We are not about shutting down and protesting. It's not a muscle that is normally flexed."