Sopa, along with the Protect Intellectual Property Act (Pipa), are two other bills being considered by members of the US Congress.

BBC: Facebook's logo

Wikipedia, which attracts millions of hits every day, is opposed to the US Stop Online Piracy Act (Sopa) and Protect Intellectual Property Act (Pipa) being debated by Congress.

BBC: Wikipedia joins blackout protest at US anti-piracy moves

Analysts say that there is a risk that the Anonymous campaign could become confused with the broader campaign against the House of Representatives' Stop Online Piracy Act (Sopa) and the Senate's Protect Intellectual Property Act (Pipa).

BBC: Hackers retaliate over Megaupload website shutdown