Wikipedia is to shut down its English-language site today in protest at draconian plans to police the internet and combat piracy.
An estimated 100million people will be affected by the one-day closure of the online encyclopaedia, which has become a vital tool for businesses and students.
Jimmy Wales, who founded the site, warned students via Twitter to ‘do your homework early’ ahead of the shutdown.
Unprecedented: The English version of Wikipedia is being blacked out for 24 hours in protest against proposed legislation currently going through Congress - the Stop Online Piracy Act and Protect Intellectual Property Act
It's part of a widespread protest campaign orchestrated via social sites such as Reddit - and other internet heavyweights such as browser company Mozilla are to join in.
They are angry about the Stop Online Piracy Act and Protect Intellectual Property Act, which are going before the U.S. Congress.
The laws are designed to prevent online pirates from making music, film, television shows and eBooks available free of charge.
However, critics argue they go much further and amount to an attempt to control and censor the internet, so curtailing freedom of expression.
The Stop Online Piracy Act and the Protect Intellectual Property Act in Congress - designed to crack down on sales of pirated U.S. products overseas - has pit internet giants, consumer groups and freedom of speech advocates against film studios and record labels.
The House bill (SOPA) would allow a private party to go straight to a website's advertising and payment providers and request they sever ties.
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 legislation could hurt the technology industry and infringe on 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 federal authorities to 'blacklist' sites that are alleged to distribute pirated content. That would essentially cut off portions of the Internet to all U.S. users. But congressional leaders appear to be backing off this provision.
Supporters say the legislation is needed to protect intellectual property and jobs. Critics say the legislation is too broad and could hurt the technology industry and infringe on free-speech rights.
There are also fears they will give the U.S. authorities even greater powers to pursue alleged law-breakers on both sides of the Atlantic.
Last week, UK courts decided to extradite British student Richard O’Dwyer to the U.S. on charges of online piracy.
In theory, the 23-year-old could be jailed for ten years for setting up the TVShack website, which provided links to free pirate downloads of films and TV programmes.
His defenders claim he is small fry in the piracy industry and say Google, which has huge financial and political muscle, is a major player.
Google has never been prosecuted even though its search engine gives links to many sites that offer illegal streaming or pirate downloads of films, music, TV shows and eBooks.
Wales said in a statement: 'Today Wikipedians from around the world have spoken about their opposition to this destructive legislation.
'This is an extraordinary action for our community to take - and while we regret having to prevent the world from having access to Wikipedia for even a second, we simply cannot ignore the fact that SOPA and PIPA endanger free speech both in the United States and abroad, and set a frightening precedent of Internet censorship for the world.'
According to a press release, users of the site have discussed for more than a month whether it should react to the legislation and, in the past few days, tried to decide how.
The foundation behind the site, Wikimedia, said it collected input from users over a period of 72 hours before making its final decision on Monday evening based on that feedback.
'This is by far the largest level of participation in a community discussion ever seen on Wikipedia, which illustrates the level of concern that Wikipedians feel about this proposed legislation,' a statement on the Wikimedia Foundation website reads.
Protesting: Wikipedia founder Jimmy Wales expects an estimated 25million daily visitors to be affected by a Wikipedia black out
Announcement: Wales wrote on Twitter yesterday that the popular community-based online encyclopedia will shut down its English versions for 24 hours in protest
'The overwhelming majority of participants support community action to encourage greater public action in response to these two bills.'
'Of the proposals considered by Wikipedians, those that would result in a "blackout" of the English Wikipedia, in concert with similar blackouts on other websites opposed to SOPA and PIPA, received the strongest support.'
Mr Wales told the BBC: ‘Proponents of the Stop Online Piracy Act 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 don’t have anything to do with stopping piracy.’
Social news community Reddit has already announced that it will 'go dark' on Wednesday to protest against the bill.
High-profile blog BoingBoing has also announced that it will protest.
Browser maker Mozilla has said that it also intends to protest by 'going dark' briefly and hosting anti-SOPA content, although copies of its Firefox browser will continue to work as normal.
Comedy network Cheezburger (host of Failblog) also said it will join in.
Several sites such as SopaStrike offer 'do it yourself' kits for smaller websites to 'go dark' in protest at SOPA.
SopaStrike hosts a list of smaller sites that have said they will black out in protest.
Not all sites who join the protest will 'switch off' entirely - some will host banners or turn the front page black instead.
Both Google and Facebook have voiced opposition to the bill but neither has committed to any form of protest this week.
Twitter will not join in - its CEO said that applying single-nation politics to a worldwide service was 'foolish'.
He claimed the proposals were part of a wider attempt by governments to regulate the internet.
‘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.’
Barack Obama has hinted he may water down the proposals, which would remove the possibility of Google and others being prosecuted for directing people to pirate websites.
Big Brother Watch, the UK civil liberties campaign group, backed the Wikipedia protest.
Its director Nick Pickles said: ‘The proposals represent a blunt attack on freedom of speech, based upon a deeply flawed understanding of how the internet works.’
A large-scale blackout is expected from midnight Eastern Standard Time on Tuesday until midnight on Wednesday.
'We are looking at a powerful protest,' said Jay Walsh, spokesman for the foundation.
Tech companies such as Google, Facebook, Yahoo and others have also questioned the legislation and said it poses a serious risk to the industry.
Several online communities such as Reddit, BoingBoing and others have announced plans to go blackout in protest.
Wikipedia is considering several different forms of response, from a banner across the top of the page to a blackout in certain areas, up to a worldwide shutdown, said Mr Walsh.
If Wikipedia opts for a blackout, it would be the largest and most well-known website to do so.
'It's not a muscle that is normally flexed,' added Mr Walsh.
As the Washington Post reports, Mr Wales expects an estimated 25million daily visitors to be affected by a Wikipedia blackout.
The Obama administration has also raised concerns about the legislation.
The administration said over the weekend that it will work with Congress on legislation to help battle piracy and counterfeiting while defending free expression, privacy, security and innovation in the Internet.