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".
Wikipedia founder Jimmy Wales: ''These bills are very badly written''
"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. (Source: BBC)