A new law imposing restrictions on users of social media has come into effect in Russia.
It means bloggers with more than 3,000 daily readers must register with the mass media regulator, Roskomnadzor, and conform to the regulations that govern the country's larger media outlets.
Internet companies will also be required to allow Russian authorities access to users' information.
One human rights group called the move "draconian".
The law was approved by Russia's upper house of parliament in April.
It includes measures to ensure that bloggers cannot remain anonymous, and states that social networks must maintain six months of data on its users.
The information must be stored on servers based in Russian territory, so that government authorities can gain access.
Critics see it as the latest in a series of recent moves to curb internet freedom.
Hugh Williamson, of New York-based Human Rights Watch, has called the law "another milestone in Russia's relentless crackdown on free expression".
"The internet is the last island of free expression in Russia and these draconian regulations are clearly aimed at putting it under government control," he added.
Opposition figures have used the internet to air their views, with some gaining millions of followers.
Commentators opposing Vladimir Putin often face restrictions in broadcast outlets and newspapers.
Analysis: Famil Ismailov, news editor, BBCRussian.com
Russian bloggers are bracing themselves for the moment when Russia's new "information security law" comes into force on 1 August. Some already share advice on how to use proxy servers in order to access social media sites that, in their view, are under threat of being closed.
It is hard to see how the law will be enforced. The servers for most of the popular social media platforms that many Russians use are based outside Russia.
Many popular bloggers are already looking for, and apparently finding, ways to "cheat" the feature that counts page visits and keep their daily unique visitor numbers just under 3000, or to make sure that the statistics are hidden altogether.
Anton Nossik, who is considered Russia's "internet guru", wrote in his LiveJournal blog that the new law didn't threaten individual bloggers directly, but provided legal grounds to block popular social networks like Facebook, Twitter, LiveJournal and Google.
"The issue of banning all these platforms in Russia is a political one and it will be decided by only one person", Mr Nossik wrote, with a thinly veiled reference to President Vladimir Putin.
Earlier in the year, Russia enacted a law that gave the government powers to block websites without explanation.
In March, Moscow blocked the blog of Mr Navalny, along with two news sites and a organisation run by Garry Kasparov - a vocal critic of the Russian government.
In a statement, Russia's prosecutor general's office said the blocks were imposed because of the sites' role in helping stage illegal protests.
Earlier this week, Twitter blocked access to an anti-Kremlin account that often publishes leaked government documents, following a request by Russia's federal communications agency Roskomnadzor.
For many years, Russia had relatively lax internet laws.
However Moscow has recently changed its tune, with Mr Putin branding the internet an ongoing "CIA project".
He also claimed that the popular Russian search engine Yandex was controlled by foreign intelligence.
Two years ago, Russia enacted a law enabling authorities to blacklist and force certain websites offline without a trial.
The government said the legislation was designed to protect children from harmful internet content, such as pro-suicide or pornography websites.
On Thursday, lawyers for US intelligence officer Edward Snowden said the whistleblower had filed for refugee status in Russia.
Mr Snowden received temporary shelter in Russia last year.
He had evaded US authorities after he leaked classified government documents revealing mass surveillance programmes undertaken by the NSA in the US and GCHQ in the UK.