The founders of Craigslist, eBay, Google, Twitter, Yahoo! and other Internet giants expressed concern to the US Congress on Wednesday over legislation intended to crack down on online piracy.

The Stop Online Piracy Act has received some bipartisan support in the House of Representatives and is the House version of a bill introduced in the Senate known as the Theft of Intellectual Property Act, or Protect IP Act.

The legislation has received the backing of Hollywood, the music industry, the Business Software Alliance, the National Association of Manufacturers, the US Chamber of Commerce and other groups.

But it has come under fire from digital rights and free speech organizations for allegedly paving the way for US authorities to shut down websites, including foreign sites, without due process and threatening the architecture of the Web.

The founders of the leading Internet companies added their voices to the chorus of opposition against the bills in an open letter to Congress published Wednesday in several US newspapers, including The Washington Post, The New York Times and The Wall Street Journal.

"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," they said in the letter.

"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," they said.

The legislation threatens to "require Web services, like the ones we helped found, to monitor what users link to, or upload," they said, warning that this would have a "chilling effect on innovation."

They said the bills would also "deny website owners the right to due process" and "give the US government the power to censor the Web using techniques similar to those used by China, Malaysia and Iran."

"We urge Congress to think hard before changing the regulation that underpins the Internet," they said. "Let's not deny the next generation of entrepreneurs and founders the same opportunities that we all had."

Signatories to the letter included 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.