Roman Rubanov, a Navalny supporter, wrote on social media that the police said they were looking into reports that there was a bomb in the office, something he said came as a surprise to him.
Hundreds of supporters of Navalny began a nationwide day of protests against the authorities on Sunday, calling on voters to boycott what they said was a rigged presidential election on March 18.
Russian police officers gather at the entrance to a business centre, which houses the office of opposition leader Alexei Navalny's Anti-corruption Foundation (FBK), in Moscow.
Russian police forced their way into the Moscow office of opposition leader Alexei Navalny on Sunday and started questioning people, images broadcast online by Navalny's supporters showed.