Russian authorities blocked access Tuesday to the website of leading Kremlin critic Alexei Navalny, the opposition activist announced on social media.
A spokesman for media regulator Roskomnadzor confirmed the move, telling TASS news agency that Russian prosecutors had ordered the restriction.
"This blockage is illegal," an indignant Navalny tweeted.
Until now Roskomnadzor had tended to periodically block pages that it considered illegal on the website, while prosecutors barred users from sharing the content.
In March 2014, Roskomnadzor blocked Navalny's blog on the popular Russian blogging platform, LiveJournal, declaring his presence on the site an infringement of his house arrest, and a violation of the ban on him using the internet and communicating with the media.
Navalny's latest post on his site, which focuses on corruption, refers to an investigation by Britain's Daily Mail paper into President Vladimir Putin's reported purchase of a Spanish villa.
In an earlier post Monday he had reiterated his call for mass demonstrations in Moscow and other big cities at the end of February.
The 38-year-old lawyer was convicted of fraud in a controversial trial late last year, and handed a suspended sentence.
His brother Oleg was jailed for three and a half years.
The pair have appealed the verdict, which the European Union called apparently "politically motivated," and the United States labelled "disturbing".
Navalny has been under house arrest since his conviction. He cut off his security bracelet on January 5 in protest over his lack of freedom.