Wikipedia has shut down its English-language site as part of protests against proposed anti-piracy laws in the US, which it has described as 'devastating to the free and open web'.

Users attempting to access the site see a black screen and a political statement: "Imagine a world without free knowledge."

The text on the black screen adds:

"For over a decade, we have spent millions of hours building the largest encyclopedia in human history. Right now, the US Congress is considering legislation that could fatally damage the free and open internet."

The site will be offline for 24 hours from midnight Eastern Standard Time (05:00 GMT) on Wednesday.

Wikipedia is opposed to the US Stop Online Piracy Act (Sopa) and Protect Intellectual Property Act (Pipa) being debated by Congress.

Last week, the news aggregation site, Reddit, announced it would shut down for 12 hours on Wednesday to express its displeasure with Sopa and Pipa.

Other sites, including the Cheezburger Network, home to massively popular Internet meme sites like I Can Has Cheezburger, have also said they will join Wednesday's "black out" protest.

The two bills that triggered the protests are backed by a wide group of copyright owners, including major record companies and Hollywood film studios, who say that online piracy has damaged their business.

However, Web companies and human rights groups have asserted that the proposed legislation will harm harm freedom of expression online and will create new tools for censorship of international websites inside the US.

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."

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, 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.