Thousands of anti-government protesters are expected to march through the Russian capital Moscow, demanding fresh elections and a new president.
The march comes after a day after police raided the homes of several prominent activists.
Last week, President Vladimir Putin signed a new law increasing fines for those who hold unsanctioned protests.
Mr Putin won a third presidential term in March despite protests over alleged fraud in December's parliamentary vote.
Tuesday's rally is the first since his return to office and correspondents say he appears to be taking a harder line against the opposition.
Those targeted by police on Monday included leading opposition activists Alexei Navalny, Sergei Udaltsov and Ilya Yashin.
Police also searched the home of Ksenia Sobchak - a well known TV presenter and daughter of Mr Putin's late mentor and St Petersburg mayor Anatoly Sobchak - who has joined the protest movement.
"People barged in at 8am, gave me no chance to get dressed, robbed the apartment, humiliated me," Ms Sobchak said in a Twitter post.
"I never thought we would return to such repression in this country."
Mr Udaltsov told reporters that police had "rifled through everything, every wardrobe, in the toilet, in the refrigerator. They searched under the beds".
Mr Navalny said police seized computer disks containing photos of his children, along with clothes including a sweatshirt bearing an opposition slogan.
Federal investigators have summoned the opposition leaders to appear for questioning just one hour before the scheduled start of the rally.
Following the raids, US State Department spokeswoman Victoria Nuland said Washington was "deeply concerned by the apparent harassment of Russian political opposition figures on the eve of the planned demonstrations on June 12".
"Taken together, these measures raise serious questions about the arbitrary use of law enforcement to stifle free speech and free assembly," she said.
The searches also triggered a wave of protest from Russian bloggers, who compared the actions to those of Stalin's secret police in the 1930s.
Artyom Liss of BBC Russian said the raids may draw new supporters to the anti-Putin cause.
MOSCOW (Reuters) - Russia's opposition expressed concern on Tuesday that Vladimir Putin was resorting to old tactics to crush dissent after riot police detained hundreds of protesters challenging the legitimacy of his presidential election victory. After three months of peaceful anti-Putin protests, police hauled away more than 500 people, including opposition leaders, who attended unsanctioned rallies in Moscow and St Petersburg on Monday or refused to go after a rally that had been permitted. ...
Police in Russia have made 550 arrests after protests against Vladimir Putin's election as president, with protest leader Alexey Navalny among those held.
They detained 300 people in central St Petersburg out of a crowd of about 800 protesters, according to local police.
A further 250 were detained in the capital Moscow, including Mr Navalny, who was later released.
Foreign states have accepted Mr Putin won on Sunday but observers said the poll had been skewed in his favour.
UK Prime Minister David Cameron phoned Mr Putin to say he was looking forward to working with him to "overcome the obstacles in the relationship between Britain and Russia and build deeper political and trade links", a statement from Downing Street said.
French President Nicolas Sarkozy congratulated Mr Putin, urging him to "continue democratic and economic modernisation".
Earlier, French Foreign Alain Juppe said the election "has not been exemplary", but added that Mr Putin's victory was not in doubt.
The US and the EU have asked the Russian government to investigate the allegations of irregularities.