The protest got the attention of some lawmakers, who did an about-face on support for the measures.
Online encyclopedia Wikipedia, the 10th most popular Web site in the United States, posted a headline on the top of every one of its pages reading, "Thank you for protecting Wikipedia." A subhead in parentheses read, "We're not done yet."
Google, which covered most of its U.S. homepage logo with a black box for much of Wednesday, said more than 4.5 million people signed its online petition to Congress, voicing displeasure with the legislation.
Twitter said more than 2.4 million posts on the proposed Stop Online Piracy Act in the House and a similar act in the Senate flowed through the site from midnight to 4 p.m. EST, some four times as many as usual.
The two bills seek to protect U.S. companies against foreign Web sites that illegally post copyrighted material. But the tech industry alleges the measures would give media companies too much power to shut down Web sites they say abuse copyrights.
The non-profit Wikimedia Foundation, which oversees Wikipedia, said 4 million people used its blacked-out U.S. site to look up contact information for their local representative.
In New York, San Francisco and Las Vegas, protesters held rallies to draw attention to the bills.
The Library of Congress said late Wednesday it had been hit with a denial-of-service attack by "a group opposed to the online piracy legislation," The Washington Post reported.
By evening, a number of lawmakers had done an about-face on the legislation.
The Senate version of the bill lost four co-sponsors, including Sen. Orrin Hatch, R-Utah.
"It is simply not ready for prime time and both sides must continue working together to find a better path forward," Hatch said in a statement.
Sens. John Boozman, R-Ark., Mark Rubio, R-Fla., and Roy Blunt, R-Mo., also said they reversed their positions and would not vote for the bill if it came up for a vote.
House Speaker John Boehner, R-Ohio, told reporters "it's pretty clear to many of us that there's a lack of consensus at this point" on how to proceed with the bill.
Bill supporters, including major media and entertainment companies, said they found it hard to get their message out.
"It's very difficult to counter the misinformation when the disseminators also own the platform," Recording Industry Association of America Chairman and Chief Executive Officer Cary Sherman was quoted in The New York Times as saying.
Facebook Chief Executive Officer Mark Zuckerberg, who had previously remained silent about the bills, posted on his own Facebook page late Wednesday afternoon, saying: "The world today needs political leaders who are pro-Internet. We have been working with many of these folks for months on better alternatives to these current proposals."
Nearly a half-million Facebook users had clicked the Like button on his post, a United Press International check early Thursday indicated.