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Thousands of sites are blocking Congress from viewing their webpages in an online demonstration against data-collection provisions of the Patriot Act.

The websites — nearly 15,000 of them as of Saturday morning — are redirecting computers from Congress to BlackOutCongress.org, where users are greeted with a stark black and white warning that reads, "We are blocking your access until you end mass surveillance laws."

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"You have conducted mass surveillance of everyone illegally and are now on record for trying to enact those programs into law," the warning continues. "You have presented Americans with the false dichotomy of reauthorizing the PATRIOT Act or passing the USA Freedom Act. The real answer is to end all authorities used to conduct mass surveillance."

The controversial provisions of the Patriot Act that allow the National Security Agency to justify bulk data collection are set to expire on Monday. The Senate must renew them by Sunday at midnight if it wants to ensure the NSA can continue legally collecting millions of phone records.

The blackout is led by Fight for the Future, an organization that describes itself as building "a grassroots movement to ensure that everyone can access the Internet’s many resources affordably, free of interference or censorship and with full privacy."

To participate in the blackout, sites embed a code snippet on their pages that detects the IP address of computers in Congress, and then redirects them to the Blackout Congress page.

Section 215 of the Patriot Act, the one that authorizes the NSA's bulk collection, was implemented after Sept. 11, 2001, to allow surveillance of domestic phone records.

Republican presidential candidate Sen. Rand Paul, who recently "filibustered" for 10 hours to object to renewing the Patriot Act, said in a statement Saturday that he plans to force the expiration of the NSA program.

"I am ready and willing to start the debate on how we fight terrorism without giving up our liberty," he said.