Wikipedia has shut down its English-language content for 24 hours in protest at anti-piracy laws being considered by the US.
Users attempting 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 today, 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.
Jimmy Wales, the founder of Wikipedia who first announced the move on his Twitter account earlier this week, said the bills were a threat to the free, open, and secure web.
"The whole thing is just a poorly designed mess," he said.