Supporters of Russian opposition leader Alexei Navalny are expected to descend on Moscow city centre for an unauthorised protest against fraud.
Mr Navalny got permission to hold the rally at another location but said he had moved it after authorities tried to "humiliate" protesters.
The prosecutor's office has warned police will take action against any unauthorised demonstrations.
Small protests are being reported in cities in the Russian Far East.
The last wave of protests led by Mr Navalny ended with hundreds arrested.
Those protests were the largest since 2012, drawing thousands of people - including many teenagers - to rallies nationwide, angered by a report Mr Navalny published accusing Prime Minister Dmitry Medvedev of corruption.
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By calling crowds into the very centre of Moscow, Alexei Navalny has set the stage for a confrontation.
The prosecutor's office has already made clear that the protest on Tverskaya Street, near the Kremlin, has not been authorised. In a statement issued well after midnight it warned that police would take action against those who break the law.Image copyright EPA Image caption Mr Navalny (centre) at a protest in Moscow in May
That could mean a repeat of scenes in March, when hundreds of people were detained during a peaceful rally against corruption - many of them, teenagers.
Navalny argues that he was forced to call people to another unauthorised rally, claiming that no firm would supply a stage, screens or speakers for the protest he had been given permission for.
So the opposition leader called for a march through central Moscow instead. It is a national holiday, and a whole series of official events are already planned on Tverskaya Street to mark Russia Day - including some military re-enactments.
Monday's protests are being seen as another test of the strength of anger towards alleged corruption at the highest levels - and a gauge of how much support Mr Navalny has for his bid to unseat President Vladimir Putin in next year's elections,
In a call for people to join him today, he wrote: "I want changes. I want to live in a modern democratic state and I want our taxes to be converted into roads, schools and hospitals, not into yachts, palaces and vineyards."
Mr Navalny, a former lawyer who was partly blinded after having a green liquid thrown in his face in April, has previously been jailed for his role in leading protests.