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.