The Moscow office of rights group Amnesty International has been sealed off by municipal officials without warning and staff cannot get inside, the group says.
Alexander Artemyev, a staff member, told Reuters that official seals had been placed on the entrances to the office, the locks had been changed, and that power to the office had been cut off.
An official notice left behind said the building was city property and that nobody was allowed to enter without being accompanied by a municipal official.
A representative of the Moscow state property department, from which Amnesty rents the office, said the department had acted within the law, but offered no immediate explanation for why the office had been sealed off on Wednesday.
John Dalhuisen, Amnesty International's Europe Director, said he did not know what was behind the decision, calling it "an unwelcome surprise."
"Given the current climate for civil society work in Russia, there are clearly any number of plausible explanations, but it's too early to draw any conclusions," Dalhuisen said in a statement.
Amnesty, which was founded in London, frequently criticises the Russian authorities over what it says are human rights violations. It said it was confident it had fulfilled all its obligations as a tenant.
Kremlin spokesman Dmitry Peskov said he was unaware of Amnesty's office problems.