Police have raided the Moscow office of an organisation founded by a top opponent of President Vladimir Putin which helps opposition candidates and political prisoners.
Open Russia said on its website that police arrived at the group's office in early afternoon and are searching it without providing an explanation.
Founder Mikhail Khodorkovsky posted a photo of camouflaged men standing in the office lobby.
There was no immediate police comment about the search.
The search came as Open Russia scheduled protest rallies against Mr Putin for Saturday.
Mr Khodorkovsky, once Russia's richest man, spent 10 years in prison on what was widely described as politicised charges before he was pardoned in December 2013 and left Russia.
While in exile, he continued to oppose Mr Putin and declared that the goal of his new movement was to make sure the Russian president does not run for a fourth term next year.
Mr Putin's spokesman, Dmitry Peskov, told reporters that authorities would react "within the framework of the law" if the unsanctioned rallies go ahead on Saturday.
Also on Thursday, unknown attackers have doused opposition leader Alexei Navalny with green antisceptic outside a conference venue in Moscow, his spokeswoman said.
Mr Navalny, who made a documentary about the Russian prime minister Dmitry Medvedev's alleged corrupt wealth which was viewed more than 20 million times online, was the key force behind nationwide anti-government rallies in March, Russia's largest and most widespread in years.