Police broke in and immediately started questioning people, it has been reported.
Photographs posted on the internet by Mr Navalny’s supporters showed about 14 policemen gathered outside his office door.
Spokeswoman Kira Yarmysh said she thought the raid was designed to shut down the work of a TV studio inside.
Hundreds of supporters of Mr 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.