Russia has blocked access to several Internet pages promoting what the authorities say is a planned illegal anti-government protest in or near Moscow's Red Square on Sunday.
The planned demonstration would take place a year before a presidential election and a week after the biggest anti-government protests in years ended in hundreds of arrests, including that of opposition leader Alexei Navalny.
Sunday's organizers describe themselves online as "young people and ordinary students from Moscow" and say they have nothing to do with Navalny, who is serving out a 15-day jail sentence for his role in organizing the March 26 protests.
As of Friday afternoon, around 2,000 people had signed up online to attend the student protest, which in the authorities' eyes is illegal because its organisers did not seek permission beforehand or agree the venue and timing with them.
A copy of what appeared to be an authentic letter from the prosecutor general's office to the country's communications regulator, Roskomnadzor, was leaked online on Friday asking for access to five Internet pages to be blocked, saying they amounted to calls for "mass disorder" and "extremist activity."
Three of those five pages were blocked Friday afternoon.
The prosecutor general's office was not immediately available for comment, but its press service confirmed to the Tass news agency it had asked for access to several pages to be blocked because they were advocating illegal protests in Moscow and "in large cities" on Sunday.
Roskomdadzor was not immediately available to comment.
President Vladimir Putin, who is expected to run for what would be a fourth term next year, spoke out against the protests Thursday, saying that anyone who broke the law should be punished.