Anti-Putin protesters have begun marching in Russia’s capital Moscow, ahead of a major rally to demand fresh elections and a new president.
The protest, 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.
Vladimir Putin won a third presidential term in March amid protests over alleged fraud in December’s parliamentary vote.
This is the first big anti-government rally in Russia since Vladimir Putin returned to the Kremlin.
There are tens of thousands of protesters, a sea of flags, banners and placards flowing through the centre of Moscow.
The demonstrators have been chanting “Putin is a thief” and “Russia without Putin”.
Vladimir Putin appears to be taking a harder line against the opposition.
Shortly before the rally, independent media websites went down with news agencies reported difficulty reaching that of the Novaya Gazeta newspaper.
Mikhail Zygar, editor-in-chief of the Dozhd (Rain) TV channel, said its website had come under attack by hackers.
“We’re trying to get back on track. The attack started at 11:00,” he told the Interfax news agency.
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.
It is a rather unsubtle attempt by the authorities to stop them from participating in the protest, our correspondent says.
Police also searched the home of Ksenia Sobchak – a well-known TV presenter and daughter of Vladimir Putin’s late mentor and St Petersburg mayor Anatoly Sobchak – who has joined the protest movement.
“People barged in at 8:00 a.m., gave me no chance to get dressed, robbed the apartment, humiliated me,” Anatoly Sobchak said in a Twitter post.
“I never thought we would return to such repression in this country.”
Sergei Udaltsov told reporters that police had “rifled through everything, every wardrobe, in the toilet, in the refrigerator. They searched under the beds”.
Alexei 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.
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.