Supporters of Russian opposition leader Alexei Navalny are expected to descend on Moscow city centre for an unauthorised protest against fraud.
Mr Navalny was given 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 that police will take action against any unauthorised demonstrations.
There have been several arrests at protests in the east of the country.
At least ten people were detained at a rally in the city of Vladivostok.
Four people were also detained in the city of Blagoveshchensk and at least one other in Kazan, as small protests got under way in cities east of Moscow ahead of nationwide demonstrations.
Mr Navalny is due to attend the rally in central Moscow later on Monday. He was earlier granted permission to hold a rally at Sakharova Avenue but changed the location on the eve of the demonstration to Tverskaya Street, near the Kremlin.
One of the groups participating in the Moscow rally, which is over government plans to demolish Soviet-era apartment blocks in the city, said it would hold its protest on Sakharova Avenue as planned.
Permission has been granted for demonstrations in 169 locations across the country, some of which will be broadcast live on the Navalny Live YouTube channel. The main rallies are expected to be in St Petersburg and Moscow.
Despite it being a public holiday in Russia, turnout in Monday's protests has so far been lower than similar rallies led by Mr Navalny in March, which led to hundreds of arrests.
Those protests were the largest since 2012, drawing thousands of people - including many teenagers - to rallies nationwide, angered by a report published by Mr Navalny that accused Prime Minister Dmitry Medvedev of corruption.
Stage set for confrontation: Sarah Rainsford, BBC News, Moscow
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 has not been authorised. In a statement issued well after midnight it warned that police would take action against those who break the law.
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.
Mr 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 on Monday, 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.
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