MOSCOW (Reuters) - Russian investigators on Saturday opened a criminal case against unidentified internet users calling for what authorities say is an illegal anti-government protest in central Moscow on Sunday.

Russia has already blocked access to several Internet pages promoting the demonstrations, which 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.

The Moscow office of Russia’s Investigative Committee, which handles the country’s most serious crimes, said it had opened the case in response to the crime of “calling for mass unrest”.

“On the Internet .., freely available to an unlimited number of people, an unidentified person using a pseudonym has posted text messages calling for actions of a severely extremist nature,” the committee said in a statement on its website.

Officials are working to establish the identity of those involved and the investigation is ongoing, it added.

President Vladimir Putin, who is expected to run for what would be a fourth term next year, spoke out against last week’s protests on Thursday, saying that anyone who broke the law should be punished.