A group of high-profile tech companies, including AOL, eBay and Facebook, have complained to Congress over a proposed bill intended to counter online piracy because they say the law may result in them being held responsible for copyright violations by their users.

The proposed House legislation the companies are unhappy with -- the Stop Online Piracy Act -- aims to introduce certain website blocking mechanisms that the companies say would effectively amount to censorship.

A similar bill, called the Protect IP Act, is also making its way through the Senate.

In a joint letter sent Tuesday to House Judiciary committee chairman Lamar Smith (R-Texas), who drafted the bill, AOL, eBay, Facebook, LinkedIn, Mozilla, Twitter, Yahoo, and Zynga said the proposed bill may "require monitoring of websites" and "pose risks" to innovation and job creation.

"Unfortunately, the bills as drafted would expose law-abiding US internet and technology companies to new uncertain liabilities, private rights of action and technology mandates that would require monitoring of websites," the letter states.

"While we work together to find additional ways to target foreign "rogue" sites, we should not jeopardize a foundation structure that has worked for content owners and internet companies alike and provides certainty to innovators with new ideas for how people create, find, discuss and share information lawfully online."

Critics of the legislation have argued that the bills could increase lawsuits against internet companies that rely on user-generated content or provide the government with too much power to shut down sites.

A congressional hearing on the issue Wednesday prompted many of the affected sites to include a "Stop Censorship" banner as part of their logos.

Tumblr, a trendy microblogging site, altered users' content streams so that they were replaced with censorship blocks. The site also displayed a link at the top of the page asking users help stop the legislation.

The company noted that Congress was considering two "well-intentioned but deeply flawed bills" that would result in the establishment of "a censorship system using the same domain blacklisting technologies pioneered by China and Iran."

Kickstarter, a site that allows users to solicit funding for their ideas and projects from its community of users, explained its opposition in a blog posting.

"If, say, someone found a single instance of copyright infringement on Kickstarter, all of Kickstarter — every project — could be taken down until it's removed," the post stated.

At Wednesday's hearing, lawmakers said they would consider changes to the legislation to address opponents' concerns, Dow Jones reported. However, they reiterated that legislation to address the issue at hand would move forward.

"Doing nothing is not an option," said Rep. Mel Watt (D-N.C.). Judiciary Committee Chairman Lamar Smith (R-Texas) agreed, saying the US could not continue with "a system that allows criminals to disregard our laws and import counterfeit and pirated goods across our physical borders."