Popular reference website Wikipedia has blacked out its pages this morning as part of a wider protest against anti-piracy legislation being debated in the US Congress.

The online encyclopaedia has shut down its English language version for 24 hours to highlight its concerns over the Stop Online Piracy Act and the Protect IP Act, which aim to strengthen copyright laws in the US.

Popular social sites like Reddit and BoingBoing also plan to shut down later, while search engine Google showed its opposition to the legislation by placing a black redaction box over its logo on its US home page.

Drafts of SOPA and PIPA have won the backing of Hollywood, the music industry, the Business Software Alliance, the National Association of Manufacturers and the US Chamber of Commerce.

However digital rights and free speech organizations claim it paves the way for US authorities to shut down websites accused of online piracy, including foreign sites, without due process.

"For over a decade, we have spent millions of hours building the largest encyclopaedia in human history," Wikipedia said in a message posted on its site at 5am, midnight on America’s east coast.

"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."

While the blackout will make it hard for people to read Wikipedia articles, its content has not gone completely offline.

The mobile version of the website will still be available to users, for example, which was left open intentionally to allow for access “in an emergency”.

The founders of Google, Twitter, Wikipedia, Yahoo! and other companies had said in a letter that the legislation would give the US government censorship powers "similar to those used by China, Malaysia and Iran."

"We oppose these bills because there are smart, targeted ways to shut down foreign rogue websites without asking American companies to censor the Internet," a Google spokesman said yesterday.

"So tomorrow we will be joining many other tech companies to highlight this issue on our US home page," the spokesman for the Internet search giant said.

The White House also expressed concern about the anti-online piracy bills in a statement over the weekend.

"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 cyber-security risk, or undermines the dynamic, innovative global Internet," it said.

"Any effort to combat online piracy must guard against the risk of online censorship of lawful activity and must not inhibit innovation by our dynamic businesses large and small," the White House said.

News Corp chief executive Rupert Murdoch, who backs the legislation, took to Twitter to accuse the "blogosphere" of "terrorizing many senators and congressmen who previously committed" to supporting it.

"Nonsense argument about danger to Internet. How about Google, others blocking porn, hate speech, etc? Internet hurt?" he wrote on the popular micro-blogging website.