Russian police on Thursday searched the offices of the Open Russia movement of former oil tycoon Mikhail Khodorkovsky, which has called a protest against President Vladimir Putin at the weekend.
The search came as three other Khodorkovsky-associated groups were put on the register of banned "undesirable" organisations for allegedly stirring unrest and undermining state security.
The Open Russia movement, which was created by the former Yukos oil company owner after his release from prison and departure into exile in 2013, openly opposes Putin's rule and seeks to rally his opponents.
The organisation's website said police seized equipment and some 100,000 flyers printed ahead of an anti-Kremlin protest dubbed "We've had enough" planned for Saturday.
Moscow police did not issue an official comment about the search, but a source told the TASS news agency that it was conducted by anti-extremism officers.
"The search has to do with the event on April 29," Open Russia coordinator Maria Baronova, who organised the protest, told journalists outside the building.
Baronova said police were "seizing all equipment and all documentation" without showing a warrant and employees remaining inside were told not to leave.
Putin's spokesman Dmitry Peskov told journalists Thursday that the Open Russia rally had not been sanctioned and warned that it would lead to "actions by the authorities."
Russia saw the biggest anti-Kremlin demonstrations in years on March 26 after opposition leader Alexei Navalny rallied thousands of protesters over high-level corruption.
More than a thousand participants were arrested at the rally in Moscow alone, with some jailed for up to 25 days.
Thursday's raid appeared to be part of a campaign by the authorities to turn the screws on groups connected with arch-Putin foe Khodorkovsky.
The justice ministry included three Khodorkovksy-linked organisations based abroad called OR (Otkrytaya Rossia), the Institute of Modern Russia and the Open Russia Civic Movement on its list of "undesirable" organisations that are banned in Russia.
However, authorities said the ban would not directly impact the similarly-named Open Russia movement based in Moscow.