Wikipedia has taken its English-language site offline as part of protests against proposed anti-piracy laws in the US.
Users attempting to access the site see a black screen and a political statement: "Imagine a world without free knowledge."
The user-generated news site Reddit and the blog Boing Boing are also taking part in the "blackout".
However, Twitter has declined to join the shutdown.
Wikipedia, which attracts millions of hits every day, is opposed to the US Stop Online Piracy Act (Sopa) and Protect Intellectual Property Act (Pipa) being debated by Congress.
The legislation would allow the Justice Department and content owners to seek court orders requiring search engines to block results associated with piracy.
The site's founder, Jimmy Wales, told the BBC: "Proponents of Sopa have characterised the opposition as being people who want to enable piracy or defend piracy".
"But that's not really the point. The point is the bill is so over broad and so badly written that it's going to impact all kinds of things that, you know, don't have anything to do with stopping piracy."
The message replacing the normal Wikipedia front page on the internet says: "For over a decade, we have spent millions of hours building the largest encyclopaedia in human history. Right now, the US Congress is considering legislation that could fatally damage the free and open internet. For 24 hours, to raise awareness, we are blacking out Wikipedia."
The site was still available on mobile phones, however.
Google.com also joined the protest, blacking out its logo and linking to an online petition urging Congress to not censor the web.
It is an unprecedented protest, says the BBC's Steve Kingstone in Washington. Analysts say it is the first major test for the young and disorganised internet industry against powerful media interests with many lobbyists in Washington.
Sopa's supporters in the House of Representatives say the legislation is designed to stop revenue flowing to "rogue websites". A similar bill, Pipa, is making its way through the US Senate.
On Saturday the White House issued a statement that appeared to side with critics of the legislation.
It said: "While we believe that online piracy by foreign websites is a serious problem that requires a serious legislative response, we will not support legislation that reduces freedom of expression, increases cybersecurity risk, or undermines the dynamic, innovative global internet."
Despite the hint of a presidential veto, Wikipedia said that the English site's administrators had decided to stage its first ever public protest because the bills "would be devastating to the free and open web".
It added: "We don't think Sopa is going away, and Pipa is still quite active. Moreover, Sopa and Pipa are just indicators of a much broader problem. All around the world, we're seeing the development of legislation intended to fight online piracy, and regulate the internet in other ways, that hurt online freedoms."
However, when asked whether Twitter would join the blackout, its chief executive, Dick Costolo, tweeted: "Closing a global business in reaction to single-issue national politics is foolish."
In a Twitter conversation with Wikipedia's founder Jimmy Wales, Mr Costolo later clarified that his comment was not meant to be read as a "value judgement" about other organisations involvement in the action.
The anti-piracy legislation still has high-profile supporters including News Corporation's chairman, Rupert Murdoch.
Over the weekend he tweeted: "So Obama has thrown in his lot with Silicon Valley paymasters who threaten all software creators with piracy, plain thievery."
He later added: "Seems like universal anger with Potus [President of the United States] from all sorts of normal supporters... Whole entertainment industry employs 2.2 million [on] average salary $65,000. Good jobs and expanding foreign earnings. Made in America, too!"
Sites taking part in the shutdown went offline for 24 hours from midnight Eastern Standard Time (05:00 GMT) on Wednesday.