The legislation is backed by Hollywood, the music industry, the Business Software Alliance, the National Association of Manufacturers, the US Chamber of Commerce and other groups.
On the oterh hand, it has come under fire from digital rights and free speech organizations for allegedly paving the way for US authorities to shut down websites without due process.
In an open letter sent on Wednesday to Congress, the founders of leading Internet companies expressed their opposition against the bills.
"We've all had the good fortune to found Internet companies and nonprofits in a regulatory climate that promotes entrepreneurship, innovation, the creation of content and free expression online.
However we're worried that the PROTECT IP Act and the Stop Online Piracy Act - which started out as well-meaning efforts to control piracy online - will undermine that framework," the letter reads.
The companies added that the proposed legislation will require web services to monitor what users link to, or upload - something that would have "a chilling effect on innovation"; it would "deny website owners the right to due process of law"; give the U.S. Government "the power to censor the web using techniques similar to those used by China, Malaysia and Iran"; and "undermine security online by changing the basic structure of the Internet."
"We urge Congress to think hard before changing the regulation that underpins the Internet. Let's not deny the next generation of entrepreneurs and founders the same opportunities that we all had," the letter concludes.
The letter - which was published Wednesday in several US newspapers, including The Washington Post, The New York Times and The Wall Street Journal - is signed by Netscape co-founder Marc Andreessen, Google co-founder Sergey Brin, Twitter co-founders Jack Dorsey, Biz Stone and Evan Williams, Flickr co-founder Caterina Fake, Yahoo! co-founders David Filo and Jerry Yang, LinkedIn co-founder Reid Hoffman, YouTube co-founder Chad Hurley, PayPal co-founder Elon Musk, Craigslist founder Craig Newmark, eBay founder Pierre Omidyar and Wikipedia founder Jimmy Wales.