LOS ANGELES - In a move that heightens the growing tension between Silicon Valley and Hollywood, Wikipedia and other websites went dark Wednesday in protest of two congressional proposals intended to thwart the online piracy of copyrighted movies and TV programs.

The web-based encyclopedia is part of a loose coalition of dot-coms and large technology companies that fear Congress is prepared to side with Hollywood and enact extreme measures - possibly including the blocking of entire websites- to stop the online sharing and unauthorized use of Hollywood productions.

The fight will test which California-based industry has the most sway in Washington.

For now, Silicon Valley appears to have the upper hand. Supporters of the legislation - called the Stop Online Piracy Act in the House and the Protect Intellectual Property Act in the Senate - say the bills are aimed at protecting jobs in the movie and music industries. But a campaign including tech heavyweights such as Google Inc. and Yahoo Inc. has successfully portrayed the bills as an attack on a free and open Internet

"It has nothing to do with stolen songs or movies," said Justin Ruben, executive director of MoveOn.org, which is participating in the blackout. Ruben says tougher legislation - even directed overseas - could make domestic cultural commentators more prone to legal attack.

Rather than showing encyclopedia articles, Wikipedia displayed a blacked-out page describing the protest and offering more information on the bills. Many articles were still viewable on cached pages.

Reddit.com shut down its social news service for 12 hours. Other sites made their views clear without cutting off services. Google blacked out the logo on its home page, directing people to a page where they could add their names to a petition.

The one-day outage was timed to coincide with key House and Senate committee hearings as they prepare to send the bills to the full floor for debate.

However, sponsor Rep. Lamar Smith, a Texas Republican, sought last week to remove a controversial provision from the House bill that could force Internet service providers to interfere with the way Web addresses work for foreign sites deemed dedicated to piracy. He postponed work on the measure until February.

Critics believe such tinkering with core Internet technology treads into dangerous territory that could lead to online censorship. It might also give hackers a new way to wreak havoc.

The White House raised concerns that the bills could stifle innovation. Over the weekend, the Obama administration reacted to two online petitions, saying it "will not support legislation that reduces freedom of expression, increases cybersecurity risk or undermines the dynamic, innovative global Internet."

At the same time, the administration called on all sides to "pass sound legislation this year that provides prosecutors and rights holders new legal tools to combat online piracy originating beyond U.S. borders."

That nuanced stance is President Barack Obama's attempt at "threading the needle" between two important constituencies as he seeks re-election in November, said Jeffrey Silva, a technology policy analyst at Medley Global Advisors.