People hold signs at a protest by the technology organization New York Tech Meetup against proposed laws to curb Internet piracy outside the offices of US Democratic Senators from New York Chuck Schumer and Kirsten Gillibrand on Third Avenue in New York.

"Right now, the US Congress is considering legislation that could fatally damage the free and open Internet," Wikipedia said in a message at its darkened website. "To raise awareness, we are blacking out Wikipedia."

Google blotted out the logo on its US home page with a black banner and published an exhortation to users to "Tell Congress: Please don't censor the Web!"

"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.

Congressional support for the legislation began to erode meanwhile in the face of the online and offline opposition -- protest rallies against the bills were held in New York and San Francisco.

A protest scheduled in Seattle was postponed due to a snowstorm.

Republican Senator Marco Rubio, a PIPA co-sponsor, said he was withdrawing his support and several other lawmakers also distanced themselves from the legislation, including influential Republican Senator Orrin Hatch of Utah.

"Congress should listen and avoid rushing through a bill that could have many unintended consequences," Rubio said on Facebook.

"After listening to concerns on both sides of the debate over #PIPA, it is simply not ready for prime time," Hatch said on Twitter.

The controversy has pitted Hollywood against Silicon Valley, forcing members of Congress to try to walk a fine line between two powerful forces, and led to an unprecedented outpouring of coordinated protest on the Web.

The Senate is to examine its bill next week, but Republican House speaker John Boehner said Wednesday there was a "lack of consensus at this point" on the House version and it would need further work in committee.

White House spokesman Jay Carney said meanwhile that the problem of online piracy needs to be tackled but "in a way which does not impinge upon a free and open Internet."

The draft legislation has won the backing of Hollywood, the music industry, entertainment giants like Rupert Murdoch's News Corp. the National Association of Manufacturers, the US Chamber of Commerce and others.

But the bills have come under fire from online companies and digital rights groups for allegedly paving the way for US authorities to shut down websites accused of online piracy, including foreign sites, without due process.

Like Wikipedia, social news site reddit also went dark, urging visitors to call their lawmaker or sign a petition, and Boing Boing took itself offline.

"These bills provide overly broad mechanisms for enforcement of copyright which would restrict innovation and threaten the existence of websites with user-submitted content," reddit said.

Blogging platform WordPress.com covered its home page with black banners with the word "censored" and Wired.com blacked out its headlines.

Other Web giants such as Twitter, Facebook and Craigslist declined to shut down for the day but voiced concern about the bills.

Twitter co-founder Jack Dorsey urged his 1.8 million followers to tweet, email and call to "tell Congress NO."

Facebook co-founder Mark Zuckerberg said the social network will "oppose any laws that will hurt the Internet."

US visitors to Craigslist were met with a message of opposition to the legislation and had to click on a link to continue on to the classifieds ad site.

Chris Dodd, head of the Motion Picture Association of America, condemned the online blackout by certain sites as an "irresponsible response and a disservice to people who rely on them for information and use their services."

"Some technology business interests are resorting to stunts that punish their users or turn them into their corporate pawns, rather than coming to the table to find solutions to a problem that all now seem to agree is very real and damaging," Dodd said.

"A so-called 'blackout' is yet another gimmick, albeit a dangerous one, designed to punish elected and administration officials who are working diligently to protect American jobs from foreign criminals," he said.