Russian authorities have searched the Moscow offices of Human Rights Watch as part of what the New York-based organisation described as a "crackdown on civil society".
Inspectors also searched the Moscow HQ of political corruption watchdog Transparency International, during a wave of raids on International NGOs.
Rachel Denber of Human Rights Watch said that officials from the prosecutor general's office and revenue inspectors conducted an "unannounced audit" and demanded documents.
"We are just the last of hundreds of NGOs that have been inspected," Denber, HRW deputy director for Europe and Central Asia, told IBTimes UK.
"It's a massive and unprecedented wave of inspections aiming to intensify the threatening atmosphere against organisations that do advocacy work on issues like human rights."
Denber said three members of the prosecutor office and one tax official searched HRW's offices for a few hours.
President Vladimir Putin has long been suspicious of NGOs and accused them of being fronts for US meddling in Russian politics. In 2012, a raft of Kremlin-backed regulatory laws requiring NGOs that received foreign funding to register as "foreign agents" was approved by Parliament.
The law was criticised by the UN High Commissioner for Human Rights and the parliamentary assembly of the Council of Europe.
Pavel Chikov, a member of the presidential human rights council, estimated that the number of NGOs searched by Russian officials was up to 2,000.
"We were kind of expecting it [the searching] given the number of other organisations that have been inspected recently," Andrew Jvirblis from Transparency International in Moscow told IBTimes UK.
"It was a bit strange however, since we had been checked by the Ministry of Justice at the end of February and they came again and asked us to provide the same documents we had already given them a month ago."
Jvirblis said inspectors were checking their law compliancy and to asses whether to demote them to foreign agent status.
"They are putting some pressure on us but we are used to it. More importantly they prevented us from working," Jvirblis said, adding the group is working on several projects including a monitoring of public officials' income declarations.
In December Putin approved a sweeping law that suspends NGOs found to be engaging in "political" activities and receiving funds from US citizens and organisations
"Any direct or indirect interference in our internal affairs, any form of pressure on Russia, on our allies and partners is inadmissible," Putin said at a meeting with the Federal Security Service.
Earlier this week it was the turn of Amnesty international and two German political NGOs (Konrad Adenauer Foundation and Friedrich Ebert Foundation) to receive a visit from prosecutors.
Germany's Foreign Minister Guido Westerwelle subsequently summoned Russia's number two diplomat in Berlin and expressed "concern over the concerted action".