Russia has introduced a law ostensibly aimed at sites containing images of child sexual abuse and other illegal material, but it has raised fears of censorship.
If offending websites cannot be shut down, the law allows authorities to force internet service providers and web hosting companies to block access.
Human rights groups fear that censorship could be extended by adding to the list of banned content.
Opponents of the law have described it yet another attempt by president Vladimir Putin to exercise control over the population.
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They see the law as providing an easy means for authorities to attack freedom of speech by blacklisting any democracy-oriented sites or any sites critical of the government.
The law has drawn protest from non-government organisations, human rights campaigners and websites, including the Russian search engine Yandex, social media portal Mail.ru and the Russian-language version of Wikipedia.