Amnesty International said Wednesday that Russian authorities had sealed off its office in Moscow and changed the locks with no warning or explanation.
"Staff found the office was sealed around 10 am (0700 GMT). The organisation had not received any warning and the premises was sealed in their absence," the rights group wrote on its website.
The global human rights campaign group said that the locks had been changed and the alarm system switched off in the premises, which is its only office in Russia.
Amnesty International regularly criticises Russian authorities for overly harsh treatment of prisoners and urges the release of those jailed in cases it views as politically motivated.
Most recently on Tuesday it urged the authorities to "end the pattern of impunity for torture and other ill treatment" after Russian activist Ildar Dadin, jailed for solo anti-Kremlin protests in Moscow, said he had been beaten and threatened in jail.
John Dalhuisen, the group's Europe director, said in a statement: "We do not know what prompted Moscow authorities to prevent our staff from accessing our offices -- an unwelcome surprise for which we received no prior warning."
He added that "given the current climate for civil society work in Russia, there are clearly any number of plausible explanations". But he said he was hopeful that there was a "simple administrative explanation".
Amnesty's office was apparently sealed off by municipal authorities, who left a piece of paper with an official stamp across the door, but Amnesty said it had been unable to reach anyone on the telephone number provided.
"We've been trying to call for two hours in a row with so far no success," the head of the Russian branch of Amnesty, Sergei Nikitin, told RIA Novosti news agency.
Nikitin said the group had been renting the premises for 20 years and had paid for the current month.
Kremlin spokesman Dmitry Peskov told journalists he did not know anything about the closure of the office.