Wikipedia has taken its English-language site offline as part of protests against proposed anti-piracy laws in the United States.
Users attempting to access the site on Wednesday see a black screen and a political statement: "Imagine a world without free knowledge".
Blogging service WordPress has removed material and Google is showing solidarity by placing a black box over its logo when US-based users visit its site, the BBC reports.
Wikipedia gets millions of online hits daily and is opposed to the US Stop Online Piracy Act (Sopa) and Protect Intellectual Property Act (Pipa) being debated by Congress.
The bills are intended to curb internet piracy by cutting off access to overseas websites illegally offering American content such as films and music.
Media firms argue that the new laws are vital to protect the movie and music industries against the theft of their products.
Hollywood also supports the bills, but Wikipedia and other internet giants like Google and Yahoo complain that the draft laws would empower the US government to censor the internet through techniques similar to those used in China and Iran.
The US bills are designed to block access to sites containing unauthorised copryight material, the BBC reports.
Content owners and the US government would be given the power to request court orders to shut down sites associated with piracy.
Advertisers, payment processors and internet service providers would be forbidden from doing business with infringers based overseas.
Sopa also requires search engines to remove foreign infringing sites from their results, a provision absent in Pipa.