The Russian-language Wikipedia website shut down Tuesday with a stark black line across its main page in protest at a bill it warned could be used to censor the Internet.
"Imagine a world without free knowledge," it said in a statement on an otherwise white page, saying amendments to be discussed in parliament Wednesday "could lead to the creation of extrajudicial censorship of the whole Russian-language Internet".
The amendments to an information law are being promoted as a crackdown on child pornography in particular, but the Ru.Wikipedia.org site warned that they could "prompt the creation of a Russian version of the Great China Firewall".
Russia already has a law "on protecting children from information that causes harm to their health and development," it said, which was used to promote age certification.
The amendments to the law, passed last week in a first reading, call for the creation of a federal register that would rule on websites carrying banned information, and oblige site owners and providers to close down the offending sites.
The legislation highlights websites carrying child pornography, promoting drug use and giving advice on suicide.
The bill is due for a second reading in the lower house of parliament on Wednesday. If passed it will go through several other votes seen as a formality before being signed into law by President Vladimir Putin.
In Russia, the Internet plays a crucial role in disseminating opposition views through social networking sites including Facebook, Twitter and Live Journal and is also used to coordinate protests.