Amnesty International’s office in Moscow has been sealed off by the city authorities, say members of staff.
Arriving for work on Wednesday, they found official seals placed across the office entrances, the locks had been changed and the power was cut off.
The human rights group says it was given no warning but hoped there was “a simple administrative explanation”.
It has in the past complained of harassment by the authorities in Russia.
Amnesty’s Moscow director, Sergei Nikitin, posted photos of the seals and discarded locks on Facebook.
Staff member Alexander Artemyev told independent Dozhd TV internet broadcaster that the organisation had been renting an office belonging to Moscow’s department of city property for 20 years.
It had always paid its rent on time, he reportedly said.
John Dalhuisen, Amnesty International’s Europe director, said: “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.
“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. We are working to resolve the situation as swiftly as possible and very much hope there is a simple administrative explanation for this setback to our work.
“We are 100% confident that we fulfilled all our obligations as tenants.”
He said staff were seeking a meeting with municipal authorities to try to resolve the situation.
President Vladimir Putin’s spokesman said he had no information about what had happened.
“This is the first time I’ve heard about it,” Dmitry Peskov told journalists, according to privately-owned Russian news agency Interfax.
Tough new laws
Laws require all NGOs receiving any overseas funding to register as foreign agents and so face restrictions.
The Russian authorities say the law is needed to protect Russia from outside attempts to influence internal politics, but many organisations complain the law is used to try to clamp down on their activities.
Last year, Russia passed a law allowing foreign organisations to be banned from operating in the country if designated as “undesirable” on national security grounds.
At the time, Mr Nikitin condemned the law as “another sobering sign of how the Russian authorities are quickly closing in on fundamental freedoms and the work of independent civil society groups in the country”.
Amnesty International’s Moscow office ‘sealed by authorities’