The two bills being considered in Congress - Stop Online Piracy Act in the House and the Protect Intellectual Property Act in the Senate - are designed to combat overseas online piracy, but websites fear they will also result in censorship of the Internet - possibly including the blocking of entire websites to stop the online sharing and unauthorized use of Hollywood productions.
Some lawmakers have indicated that the bills will be modified to address those concerns. The bills intend on protecting jobs in the movie and music industries.
A campaign that includes tech heavyweights Google Inc. and Yahoo Inc. has successfully portrayed the bills as an attack on a free and open Internet.
The bills also make companies like Wikipedia more liable if illegal content is posted on their sites.
"The reason they are so concerned about this, it could cost them millions of dollars in terms of compliance, implementing new technology and ultimately putting them on the hook for any infringement users commit," said Jack Lerner, professor at the USC Gould School of Law.
The English language version of Wikipedia was replaced with a message that explains the reason for the blackout. They claim 162 million people saw their message, shut down Congress' switch boards and melted their servers.
Reddit.com shut down its social news service for 12 hours. Other sites made their views clear without cutting off surfers.
Google blacked out the logo on its home page, directing surfers to a page where they could add their names to a petition against the bills.
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.
Amid the high-tech campaign against the bills, several lawmakers came out in opposition. At least four Senate Republicans who had previously cosponsored the Senate bill - Orrin Hatch of Utah, Roy Blunt of Missouri, John Boozman of Arkansas and Charles Grassley of Iowa - issued statements Wednesday saying they were withdrawing their support. Democratic Sen. Ben Cardin of Maryland last week said that, after listening to constituent concerns, he could not vote for the Senate bill as it is currently written.
On the House side, Rep. Rick Larsen, D-Wash., issued a statement that he had heard from many of his constituents and come to the conclusion that the House and Senate bills "create unacceptable threats to free speech and free access to the Internet."
Republican co-sponsor Marco Rubio is backing away from it. He posted on his Facebook what that he has heard "legitimate concerns."
California's Democratic senators Dianne Feinstein and Barbara Boxer remain as co-sponsors of the bill.
The Associated Press contributed to this report.