The Russian-language Wikipedia website shut down Tuesday and symbolically blacked out its logo in protest at a bill that would allow the state to block access to blacklisted websites.
"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 existing 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 Firewall of China."
The amendments, passed Friday 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 bill comes as part of an apparent trend to use the pliant Duma lower house dominated by the ruling United Russia party to rubberstamp laws that can be used against the opposition.
It has recently pushed through legislation ramping up fines for protesters and stigmatising internationally-funded NGOs as "foreign agents."
The legislation highlights websites carrying child pornography, promoting drug use and encouraging children to commit suicide, saying the decision to blacklist these would be taken by a federal agency.
It would also blacklist sites using Russia's vague extremism laws, which can be enforced by the ruling of any district court.
It also provides for a whole site to be blacklisted over the content of one page.
The bill was proposed by the Duma's family, women and children committee, an all-party group.
It has already prompted high-level opposition.
The presidential council on human rights — a purely advisory body — last week slammed the bill, calling the measures "the introduction of censorship" and a "new electronic curtain" descending on Russia.
And the newly appointed minister of communications, Nikolai Nikiforov, Russia's youngest at 30, took the unusual step on Tuesday of criticising it on Twitter.
"I do not support Wiki's decision to close. But this step is an important reaction from society, a sign that we need to amend the bill," he wrote.
"The idea of fighting child pornography on the net is correct. But the Internet as a whole must remain a free environment."
Nevertheless, he predicted that the bill would be passed in its crucial second reading, saying that industry representatives and experts should work over the summer on amendments to propose in the autumn.
Prime Minister Dmitry Medvedev, who broke the mould by using social media as president, has also criticised the bill, Vedomosti business daily reported Tuesday citing a source in a major internet company.
Yet President Vladimir Putin is believed to rarely use the Internet and has called it "50 percent porn."
In neighbouring Belarus, strongman leader Alexander Lukashenko has cracked down on the Internet, creating a special centre that monitors those who access suspicious sites and requiring Internet cafes to keep records of users.
The bill is expected to have its second reading in the Duma on Wednesday.
If passed it will go through several other votes seen as a formality before being signed into law by Putin. It would come into force from January 1 next year.
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.