The online encyclopedia went dark at midnight ET with the message, “Imagine a world without free knowledge.”

“For over a decade, we have spent millions of hours building the largest encyclopedia in human history. Right now, the U.S. 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 then directs users to contact their representative. It also provides a link to Wikipedia’s SOPA and PIPA page.

The blackout applied only to the English version of the site.

Update: Google’s protest also started at midnight, but instead of going dark the search engine covered its logo in black. The simple message: “Tell Congress: Please don’t censor the web.”

More features of Wednesday’s blackout: Huffington Post features a giant black box where its lead story normally is displayed. Boing Boing warned of “legal jeopardy” sites would face if the legislation passed. Reddit plans to go dark at 8 a.m. ET

Also on the protest: WordPress and Craigslist, as well as Wired.