Gudkov said he believes the investigation was aimed at preventing him from running for the national parliament this year. He was a parliament member in 2011-16.
Gudkov was detained at his country house last week in connection with an investigation into money allegedly owed to the city for rental of office space. He was released without charges Thursday.
Last week, a court sent prominent opposition figure Andrei Pivovarov to jail for two months pending a probe into his actions as head of the Open Russia group, which authorities have declared an undesirable organization. Pivovarov had announced the group was disbanding several days before his arrest.