Support for two online piracy bills in Congress appeared to wane Wednesday after opponents of the legislation staged a dramatic protest in which vast swaths of the Web effectively went dark.

Visitors to Wikipedia who tried to search the online encyclopedia’s usually trivia-filled pages were instead greeted by a message informing them that the bills could “fatally damage the free and open Internet.” On Craigslist, those looking to search the classifieds had to first read through a note urging them to contact their representatives to block the bills. And while you could still run searches on Google, a black censorship bar blocked the area where a cheery Google Doodle logo normally resides.

The search engine directed users to a petition opposing the bills.

More than 4.5 million people signed their names to the Google petition and 300,000 people emailed or called their lawmakers, according to the protest organizers. In New York, San Francisco and Las Vegas, protesters held rallies to draw attention to the bills. The Library of Congress said late Wednesday that it had been hit with a denial of service attack by “a group opposed to the online piracy legislation.”

By the evening, a number of lawmakers had done an about-face on the legislation.

The Senate version of the bill lost four of its 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 about the Protect Intellectual Property Act.

Sens. John Boozman (R-Ark.), Mark Rubio (R-Fla.) and Roy Blunt (R-Mo.) also released statements Wednesday saying that they had reservations and would not vote for the bill if it came up for a floor vote.

In the House, where lawmakers are considering a similar bill called the Stop Online Piracy Act, or SOPA, House Speaker John Boehner (R-Ohio) told reporters that “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.

The online piracy bills had been aimed at protecting U.S. companies against foreign Web sites that illegally post copyrighted material. Companies opposing the legislation had argued that the bills would impose heavy regulatory costs, harm innovation and give the government too much power to shut down Web sites accused of copyright violations even if they are later found to be innocent of the charges.

“The entire approach is philosophically wrongheaded,” Wikipedia founder Jimmy Wales said Tuesday evening in an interview with The Washington Post before the protest began.

In a statement posted to his public Facebook profile, co-founder and chief executive Mark Zuckerberg said the bills “get in the way of the internet’s development.” Google’s chief executive, Eric Schmidt encouraged his followers on Twitter to sign Google’s petition against the bills, calling on them to “Defend the web!”

The darkened Web sites represent some of the largest properties on the Internet: Google easily has the widest reach, with 187.1 million unique visitors in December, according to data from ComScore. Wikimedia, which owns Wikipedia and other wiki sites, and Craigslist also have broad audiences, reaching 83 million and 49.8 million unique visitors, respectively, in the same period. Reddit, which compiles links to funny stories, was visited by 4.8 million users last month. Another participant, Boing Boing, had 1.6 million visits.

Sen. Patrick Leahy (D-Vt.), the Senate bill’s lead sponsor, on Tuesday had steadfastly defended the bill, arguing that the Web firms involved in the protests were overreacting. Much of what has been claimed about the the bills is “flatly wrong and seems intended more to stoke fear and concern than to shed light or foster workable solutions,” he said. Leahy did not comment Wednesday.

Staff writer Felicia Somnez contributed to this report.