Two Russian rights groups' offices were spray-painted with the words "foreign agent" on Wednesday, the day a Kremlin-backed law on foreign-funded non-governmental organisations came into effect.
Much criticised by the West, the law forces foreign-funded NGOs engaging in "political activities" to register themselves using the politically loaded term "foreign agent" that echoes Cold War-era hostilities.
The Kremlin, which denies orchestrating a clamp-down on dissent, has said the law is needed to tighten control over foreign-funded organisations operating in Russia to prevent them gathering intelligence for other governments.
The red door leading to the Moscow offices of rights group For Human Rights was spray-painted with the words and a heart with "USA" written next to it, and similar graffiti was sprayed on the wall of offices used by rights group, Memorial.
It was not clear who was responsible for the vandalism.
Both NGOs are among a group that plans to boycott the law.
"We will not follow this law, it is unlawful," For Human Rights director Lev Ponomaryov told Reuters. He said his organisation had notified the police about the graffiti.
Ponomaryov said he did not know who had sprayed the door, but that the law was a scare tactic to try to restrict the operations of organisations like his.
"We have to show some sort of civil disobedience. They are threatening us and if they apply some sort of repressions to us, we will try to make our case in courts," he said.
Groups violating the law may be fined or suspended for up to six months without a trial. Rights activists say the law violates Russia's constitution and the right to associate and assemble freely.
Several leading rights groups have decided to challenge the law at the European Court of Human Rights.
They say the NGO law is one in a series of repressive bills rushed through the Russian parliament since President Vladimir Putin's third term began in May.
Putin's victory in the March presidential election spawned widespread accusations of fraud and large street protests.