The Russian-language Wikipedia website shut down for a day 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 its otherwise white main 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".
Passed in its first reading on Friday, the bill calls 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.
"It is unclear who would control the activities of this organisation," the head of Russian Wikipedia, Vladimir Medeiko, complained to the RIA Novosti news agency.
"There is a great risk of censorship appearing on the Russian Internet."
In a storm of comment, Russia's main Internet companies including Yandex and Mail.ru Group backed Wikipedia's objections to the bill.
The bill comes as part of an apparent trend to use the pliant Duma lower house dominated by the ruling United Russia party to rubber-stamp 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".
Protest leader Alexei Navalny slammed the bill on his popular blog saying it would "move the Internet in the direction of the Zombie Box," a slang term for Kremlin-controlled television.
The bill cites websites carrying child pornography, promoting drug use and encouraging children to commit suicide, saying the decision to blacklist these would be taken by an unnamed federal agency.
It said that sites would also be blacklisted under vague extremism laws, which can be enforced by the ruling of a single district court.
Under the bill, an entire site could 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 it "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 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."
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.
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.
In neighbouring Belarus, strongman leader Alexander Lukashenko has cracked down on the Internet, creating a centre to monitor those who access suspicious sites and requiring Internet cafes to keep records of users.
The bill's second reading is timetabled for 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.