Anti-government protesters are gathering in the Russian capital Moscow, for a major rally to demand fresh elections and a new president.
The march, on a national holiday, comes a day after police raided the homes of several prominent activists.
They were all ordered to report for questioning on Tuesday, and so were likely to miss the march.
Last week, President Vladimir Putin signed a new law increasing fines for those who violate protest laws.
Putin won a third presidential term in March amid 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.
Shortly before the rally independent media websites went down. The BBC could not access the Moscow Echo radio website, and other news agencies reported difficulty reaching that of the Novaya Gazeta newspaper.
Those targeted by police on Monday included leading opposition activists Alexei Navalny, Sergei Udaltsov and his wife Anastasia.
They all arrived for questioning at the headquarters of the Russian investigative committee on Tuesday morning.
The BBC’s Steve Rosenberg in Moscow said it was a rather unsubtle attempt by the authorities to stop them from participating in the protest.
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.
In a separate development in Warsaw, thousands of Russian fans are due to mark their national holiday with a march through the city ahead of their Euro 2012 match against co-host Poland.
It will be heavily policed in what the authorities say is the “greatest ever” security challenge.
