Updated

Police in Moscow have issued a warning to people intending to take part in an anti-Kremlin march organized by opposition leader Alexei Navalny.

"Your personal safety may be under threat," said a statement posted Thursday on the website of Moscow's police department. "The event is illegal."

More than 4,000 people have said on Facebook that they will attend the march scheduled for Sunday, which is protesting against high-level corruption.

Navalny, who is campaigning to run in Russia's 2018 presidential election, wrote Thursday on his blog that "the law is on our side."

The organizers of largely peaceful opposition gatherings in Moscow in recent years have always agreed in advance with city authorities about a venue. A lack of agreement this time raises the possibility of mass arrests.