At the stroke of midnight Wednesday, large swaths of content that Web surfers have gotten used to looking up every day started disappearing.

Visitors to Wikipedia who are trying to search the encyclopedia’s usual trivia-filled pages are instead greeted by a shadowy “W” and a message saying, “Imagine a World Without Free Knowledge” — unless they type in the words SOPA or Protect IP ACT.

Craigslist isn’t imposing a blackout per se, but before searchers can access the classified listings they’re hit with a message asking them to tell their senators and congressmen to oppose two online piracy bills working their way through Capitol Hill.

Related: How to get around the blackout

The online blackout is part of a protest by popular Internet sites against the House’s Stop Online Piracy Act (SOPA) and the Senate’s Protect Intellectual Property Act (PIPA). Companies argue 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.

If you fire up Google’s home page, you’ll also notice a change. Instead of a cheery Google Doodle, a black censorship bar has been hastily thrown over the logo (users can actually see the wrinkles). The message “Tell Congress: Please don’t censor the Web” is displayed under the search box. Click on the blackened logo or the link, and it directs you to a history of the anti-SOPA and anti-PIPA movement.

Head to the social news site Reddit, and you’ll see only a page of information on the bills that tells how to contact your local member Congress and gives updates on the live protests the Web site is staging across the country.

The twin bills were drafted to target foreign Web sites that illegally post copyrighted material from the United States. But these Web firms argue that the onus of blocking out pirated material rests on U.S. companies — search engines, aggregators and forums — who worry they’ll have to take on the role of policing every link on their Web sites.

Piracy is a problem, most of the companies acknowledge, but they say that these bills are too vague and don’t define who is responsible for making sure pirated material based on foreign sites aren’t showing up on American Web pages.

Wales said that Congress should go after advertising and payments in dollars flowing to these sites rather than asking aggregators and others to sanitize their content.

“A lot of the provisions are much too loose. They need to have more protections for innocent sites,” he said.

Internet companies have been fighting the bill for months, and on Friday Rep. Lamar Smith (R-Texas), the sponsor of the House bill, offered a concession. He said he would drop the most controversial portion of SOPA, which would have blocked traffic to offending sites. In the Senate, Patrick Leahy (D-Vt.) has said he will consider a similar amendment to PIPA if the bill moves to the Senate floor. A cloture vote is scheduled in the Senate for Jan. 24.

The uproar has been so great that the White House weighed in on the debate on Saturday with a statement that expressed concern about the piracy bills.

Wales said he feels the changes offered by the legislators are not enough to make Wikipedia and other Internet companies support the bill.

“I think it would be better to burn them down and start over,” he said

The SOPA Strike was organized by the nonprofit Fight for The Future, which claims to have more than 10,000 participating Web sites taking part in the effort.

Reddit was the first to announce their plans to go dark last week, but other popular Web sites soon joined them. Mozilla, the nonprofit organization that publishes the Firefox browser, blacked out their U.S. Web pages. TwitPic has a black bar over its logo, as well; WordPress has covered all of its in black. has gone white — with a tongue-in-cheek message that calls the bills a “perfect excuse” to stop collecting news.

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

But there are plenty of other sites that can keep a Web addict’s attention throughout the day: You can still buy and sell on eBay, watch a YouTube video and broadcast your frustration over the blackout to your friends and followers on Twitter and Facebook.

Related stories:

SOPA: Getting around the blackout

Basulto | Wikipedia goes dark, but is the site relevant?

Google to state anti-SOPA stance on home page

SOPA protests shut down Web sites

How site blackouts will affect your daily search