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The legislation, which is being pushed by media owners such as Rupert Murdoch, is intended to make it easier for US owners of media such as films, TV shows and music to cut off sites giving illegal access to their material.
Sites such as Wikipedia claim the legislation would also place unfair and unrealistic responsibilities on them to check no material on their site was copyrighted.
The White House has already indicated that it will not accept parts of the act, but Wikipedia founder Jimmy Wales is going ahead with the protest anyway.
He explained that the site will be replaced by a message urging users to protest to Congress, and said: "My goal is to melt switchboards!
"We have no indication that SOPA is fully off the table. We need to send Washington a BIG message."
The move is also opposed by link-sharing site Reddit, which will be joining the protest, and search titan Google, which will not (presumably in case society falls apart).
Mr Wales is also concerned about the companion legislation, the Protect Intellectual Property Act (PIPA), saying "PIPA is still extremely dangerous".
And he warned students to get ahead on their studies before the switch off, from 5AM GMT tomorrow until the same time on Thursday, telling them to "do your homework early".
Rupert Murdoch has expressed his displeasure at the White House's stance on the issue, tweeting that President Obama "has thrown in his lot with Silicon Valley paymasters".
What do you think? Will you survive without Wikipedia? Is it an admirable gesture? Tell us in the comments below...