Thousands of internet sites are taking part in a "blackout" protest against anti-piracy laws being discussed by US lawmakers.
The Wikipedia encyclopedia and blogging service WordPress are among the highest profile pages to remove material.
Google is showing solidarity by placing a black box over its logo when US-based users visit its site.
The Motion Picture Association of America has branded the action as "irresponsible" and a "stunt".
Visitors to Wikipedia's English-language site are greeted by a dark page with white text that says: "Imagine a world without free knowledge... 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."
It provides a link to more details about the House of Representatives' Stop Online Piracy Act (Sopa) and the Senate's Protect Intellectual Property Act (Pipa).
If users try to access its other pages via search sites, the text briefly flashes up before being replaced by the protest page. However, people have been sharing workarounds to disable the redirect.
WordPress's homepage displays a video which claims that Sopa "breaks the internet" and asks users to add their name to a petition asking Congress to stop the bill.
"The authors of the legislation don't seem to really understand how the internet works," the site's co-founder, Matt Mullenweg told the BBC.
Across the globe, several Pirate Party sites have been taken offline. The political parties - which advocate reform of copyright laws - took the action in the UK, Spain, Sweden, Argentina, Canada and elsewhere.
The news recommendation site Reddit, the online magazine Boing Boing, the software download service Tucows and the German hackers' group the Chaos Computer Congress also removed access to their content.
The tech news site Wired covered its headlines and pictures with black boxes which were only removed when covered with the cursor.
The US news website Politico estimated that 7,000 sites were involved by early Wednesday morning.
The moves were described as an "abuse of power" by one of the highest profile supporters of the anti-piracy bills.
"Some technology business interests are resorting to stunts that punish their users or turn them into their corporate pawns, rather than coming to the table to find solutions to a problem that all now seem to agree is very real and damaging," said former Senator Chris Dodd, the chairman of the Motion Picture Association of America.
"It is an irresponsible response and a disservice to people who rely on them for information... A so-called 'blackout' is yet another gimmick, albeit a dangerous one, designed to punish elected and administration officials who are working diligently to protect American jobs from foreign criminals."
The US Chamber of Commerce said that the claims against the legislation had been overstated.
"[The sponsors] announced they would roll back the provisions of these bills designed to block foreign criminal websites, striking a major conciliatory note with those who raised legitimate concerns," said Steve Tepp, chief intellectual property counsel at the chamber's Global Intellectual Property Center.
"That was on top of the changes that guarantee the bill applies only to foreign sites. What remains are two pieces of legislation that are narrowly tailored and commercially reasonable for taking an effective swipe at the business models of rogue sites."
The proposed legislation would allow the Department of Justice and content owners to seek court orders against any site accused of "enabling or facilitating" piracy.
Sopa also calls for search engines to remove infringing sites from their results. Pipa does not include this provision.
'Threat to innovation'
Google posted a blog on the subject claiming that thebills would not stop piracy.
"Pirate sites would just change their addresses in order to continue their criminal activities," it said.
"There are better ways to address piracy than to ask US companies to censor the internet. The foreign rogue sites are in it for the money, and we believe the best way to shut them down is to cut off their sources of funding."
Other net firms that have criticised the legislation decided not to take part in the blackout.
Twitter's founder, Dick Costolo, tweeted that it would be"foolish" to take the service offline.
Facebook declined to comment on the page blackouts but referred users to anew page posted by its Washington DC divisionwhich said: "The bills contain overly broad definitions and create a new private cause of action against companies on the basis of those expansive definitions, which could seriously hamper the innovation, growth, and investment in new companies that have been the hallmarks of the internet."
The events coincided with news that the US House of Representatives plans to resume work on Sopa next month.
The chairman of the House Judiciary Committee, Lamar Smith, said: "I am committed to continuing to work with my colleagues in the House and Senate to send a bipartisan bill to the White House."
The Senate is expected to start voting on 24 January on how to proceed on Pipa.
Even if Congress approves the bills, President Barack Obama may decide to veto them.
The White Houseissued a statementat the weekend saying that "we will not support legislation that reduces freedom of expression, increases cybersecurity risk, or undermines the dynamic, innovative global internet".