US congressional support for legislation targeting online piracy was eroding Wednesday as Wikipedia and Google led an online onslaught and branded the bills a danger to Internet freedom.

Hundreds of other websites joined the Web titans in denouncing the bills introduced in the House of Representatives and the Senate aimed at cracking down on online piracy of movies and music and the sale of counterfeit goods.

Wikipedia shut down the English-language version of its online encyclopedia for 24 hours to protest the legislation -- the Stop Online Piracy Act (SOPA) in the House and the Protect IP Act (PIPA) in the Senate.

"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!"

Congressional backing for the legislation was splintering in the face of the online and offline opposition -- protest rallies against the bills were held in New York and San Francisco.

Republican Senator Marco Rubio, a PIPA co-sponsor, said he was withdrawing his support and several other lawmakers 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.

"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 the problem of online piracy needs to be tackled but "in a way which does not impinge upon a free and open Internet."

The White House said two petitions opposing the bills had drawn over 100,000 signatures.

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.

"We think that there are no due process protections that would protect us from being shut down," Tumblr executive vice president Andrew McLaughlin said at the New York rally.

"Just because one of our users on Tumblr -- which has, you know, more than 40 million blogs -- if one of them did something illegal it could lead to an entire site getting shut down," McLaughlin said.

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.

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 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."

Craigslist visitors 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.