Wikipedia and a host of other internet companies companies blacked out their websites in one way or the other early this morning in protest against controversial legislation in the US.

Google blacked out its logo and posted a message on its home page that said "Tell Congress: Please don't censor the Web!". The message, which was visible to users of outside the US as well, linked to a petition opposing the Stop Online Piracy Act (SOPA) in the US House of Representatives, and the Protect IP Act (PIPA) in the US Senate.

More websites are expected to register their protest by 8:00am Eastern Time today, 1pm in the UK.

Ben Huh, CEO of Cheezburger, which runs humour websites, for example, said in a Twitter message that his company sites would have a blackout from 8.00am.

Wikimedia Foundation said earlier this week that the Wikipedia community had decided to black out its English version to users worldwide to protest against SOPA and PIPA.

In a message titled, "Imagine a World Without Free Knowledge", the online encyclopedia said on its English site after the black out at midnight that the US Congress is considering legislation "that could fatally damage the free and open internet". It said it was blacking out Wikipedia for 24 hours to raise awareness.

On Saturday, three officials with US President Barack Obama's administration issued a statement that appeared to oppose SOPA and PIPA. The officials said that online piracy by foreign websites is a serious problem that requires a serious legislative response, but the administration would not support legislation that reduces freedom of expression, increases cybersecurity risk, or undermines the dynamic, innovative global internet.

Jimmy Wales, cofounder of Wikipedia, warned on Twitter on Monday that rumors of the death of SOPA may be premature, and PIPA is still going strong.

US Representative Lamar Smith, the lead sponsor of SOPA and chairman of the Judiciary Committee, said Tuesday he intends to resume the markup session next month. In a markup hearing, a committee votes on amendments to a bill and votes on whether to approve the bill and send it to the full House for action.

Activist group Electronic Frontier Foundation on Wednesday superimposed a banner "Stop Censorship" on its logo and blacked out the background of its website.

Some websites did not shut down access to other pages like Wikipedia did, and only displayed a splash screen on the home page.

Craigslist appeared to have blacked out only its US sites. "Imagine a world without craigslist, Wikipedia, Google, [your favorite sites here]...." it said in a splash screen on its website, while claiming that media companies like Sony and News Corp. want to make "that world your reality". It warned "corporate paymasters" to keep their "clammy hands" off the Internet.

Hacker group Anonymous also joined in the protest by posting messages against SOPA on its websites.

Social news site Reddit said on its website that it would be closing down the site for 12 hours to protest SOPA and PIPA. It had said earlier that it would be closing down the site from 8:00am Eastern time, 1pm in the UK.

Twitter has however not participated in the protests. "Not shutting down a service doesn't equal not taking the proper stance on an issue", said Dick Costolo, CEO of Twitter, who in an earlier Twitter message had said that "closing a global business in reaction to single-issue national politics" was foolish.