Police armed with assault rifles raided the homes of Russia's top protest leaders today in an unusually blunt show of force on the eve of a mass rally against President Vladimir Putin's rule.
The coordinated security sweep in the early hours of a public holiday targeted the homes of a new brand of young Russian politicians who analysts believe represent the biggest threat to ex-KGB spy Putin's 12-year rule.
Some of their supporters compared the raids to Soviet dictator Joseph Stalin's night-time arrests of his biggest foes during the Red Terror wave of the 1930s.
And even Putin's own human rights council adviser expressed shock at what to many appeared to be a blatant and previously unseen campaign to intimidate the Kremlin's biggest foes into submission.
"I think that from the standpoint of social harmony, modernisation and political reforms, this is the very worst that could have happened," adviser Mikhail Fedotov told Interfax.
Washington was "deeply concerned by the apparent harassment of Russian political opposition figures on the eve of the planned demonstrations on June 12," State Department spokeswoman Victoria Nuland told reporters in the US capital.
Officers beat down the doors of the increasingly popular anti-corruption blogger Alexei Navalny as well as media celebrity Ksenya Sobchak, a more recent Putin critic.
Others on the list included Sergei Udaltsov -- an outspoken ultra-leftist who stages periodic hunger strikes to protest his repeated arrests -- and the far more moderate democracy campaigner Ilya Yashin.
"They are taking all the electronic devices," Navalny tweeted during the raid. "Even disks with photos of the children."
Russia's powerful Investigative Committee said 10 raids were conducted in all as part of a probe over a May 6 demonstration "that ended in mass disturbances."
A so-called "March of Millions" that drew 20,000 people in Moscow ended in the arrest of hundreds after bloody clashes broke out between protesters and police on the eve of Putin's inauguration to a third term.
Navalny and the nine others face up to 10 years in prison if they are charged and convicted of organising mass disturbances.
The May unrest sparked a stiff response from the Kremlin. On Friday, Putin signed into law legislation dramatically raising fines for those who break the already restrictive laws on organising and holding rallies.
The highest penalty for individuals has been raised to 300,000 rubles ($9,000) -- more than for any other administrative offence and roughly equivalent to Russians' average annual salary.
Rights activists said the security agencies were trying to intimidate the nascent movement and prevent mass attendance at Tuesday's protest in order to avoid embarrassing Putin just a month into his historic third term.
"This recalls 1937," campaigner Valery Borshchyov said in reference to the most infamous year of Stalin's repression.
"In those days, they also made the arrests at night, in secret, when people were not working," he told Interfax.
The Investigative Committee has ordered all those targeted to appear for questioning just an hour before the planned start of Tuesday's march.
Former finance chief Alexei Kudrin -- a personal friend of Putin who quit last year after disputing the president's job swap with current Prime Minister Dmitry Medvedev -- said the raids showed "radicals gaining strength" in the Kremlin.
Sobchak said she did not attend the May protest but was still being investigated for potentially providing illegal funding to the movement.
"It turns out that in this country, not only are you kept from expressing your free opinion, but you are also not allowed to talk to people who do not suit the state," Sobchak told reporters outside her apartment building.
Udaltsov for his part said police had spent six hours searching through his apartment and eventually walked away with his computer hard drive and various memory devices.
"They also searched my parents' apartment ... but did not find anything interesting," he told Interfax.