The sites' webmasters are opposed to the Stop Online Piracy Act (Sopa) and Protect Intellectual Property Act (Pipa) being debated by Congress.
However, Twitter has declined to take part in the shutdown.
Wikipedia'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."
Veto hint Sopa's supporters in the House of Representatives say the legislation is designed to stop revenue flowing to "rogue websites".
It would give content owners and the US government the power to request court orders to shut down sites associated with piracy.
It could also force US-based internet service providers, search engines, payment processors and advertising networks to stop doing business or offering links to foreign sites suspected of involvement with the illegal activity.
A similar law, 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 Acts.
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."
'Broader problem' 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 plan to go offline for 24 hours from midnight Eastern Standard Time (05:00 GMT) on Wednesday.
From other news sites
V3.co.uk Sites prepare to go dark in SOPA protest 3 hrs ago
Los Angeles Times Wikipedia to go offline to protest anti-piracy legislation 4 hrs ago
ZDNet UK Wikipedia to turn off for 24 hours in SOPA protest 14 hrs ago
Total Telecom Internet giants threaten 'blackout' protest against SOPA 13 Jan 2012
Techworld.com Reddit blackout looms in SOPA protest 13 Jan 2012