Wikipedia went dark, Google blotted out its logo and other popular websites planned protests to voice concern over legislation in the US Congress intended to crack down on online piracy.
Wikipedia tonight shut down the English version of its online encyclopaedia for 24 hours to protest the Stop Online Piracy Act (SOPA) introduced in the House of Representatives and the Senate version, the Protect IP Act (PIPA).
Google placed a black redaction box over the logo on its much-visited US home page to draw attention to the bills, while social news site reddit and the popular Cheezburger humour network planned to shut down later in the day.
The draft legislation has won the backing of Hollywood, the music industry, the Business Software Alliance, the National Association of Manufacturers and the US Chamber of Commerce.
But it has come under fire from digital rights and free speech organisations for allegedly paving the way for US authorities to shut down websites accused of online piracy, including foreign sites, without due process.
"For over a decade, we have spent millions of hours building the largest encyclopaedia in human history," Wikipedia said in a message posted at midnight (1600 AEDT) on its darkened website.
"Right now, the US Congress is considering legislation that could fatally damage the free and open internet. For 24 hours, to raise awareness, we are blacking out Wikipedia."
The founders of Google, Twitter, Wikipedia, Yahoo! and other internet giants said in an open letter last month the legislation would give the US government censorship powers "similar to those used by China, Malaysia and Iran."
"We oppose these bills because there are smart, targeted ways to shut down foreign rogue websites without asking American companies to censor the internet," a Google spokesman said on Tuesday.
"So tomorrow we will be joining many other tech companies to highlight this issue on our US home page," the spokesman for the internet search giant said.
Reddit said it will shut down for 12 hours to protest the legislation.
"We wouldn't do this if we didn't believe this legislation and the forces behind it were a serious threat to reddit and the internet as we know it," reddit said.
"The freedom, innovation, and economic opportunity that the internet enables is in jeopardy."
Ben Huh, the founder of Cheezburger network, said on his Twitter feed that his 58 sites, which include icanhascheezburger.com, FAIL Blog and The Daily What, will observe a blackout.
Wikipedia founder Jimmy Wales announced the plans to shut down the site in a message on his Twitter feed.
"Student warning! Do your homework early. Wikipedia protesting bad law on Wednesday!" Wales said.
"This is going to be wow. I hope Wikipedia will melt phone systems in Washington on Wednesday. Tell everyone you know!" he said.
Volunteer-staffed Wikipedia turned 11 years old on January 15 and boasts more than 20 million articles in 282 languages.
The White House expressed concern about the anti-online piracy bills in a statement over the weekend.
"While we believe that online piracy by foreign websites is a serious problem that requires a serious legislative response, we will not support legislation that reduces freedom of expression, increases cybersecurity risk, or undermines the dynamic, innovative global internet," it said.
"Any effort to combat online piracy must guard against the risk of online censorship of lawful activity and must not inhibit innovation by our dynamic businesses large and small," the White House said.
News Corp chief executive Rupert Murdoch, who backs the US legislation, accused the "blogosphere" of "terrorising many senators and congressmen who previously committed" to supporting it.
"Nonsense argument about danger to internet. How about Google, others blocking porn, hate speech, etc? internet hurt?" he wrote on the popular micro-blogging website.