The rally, a response by pro-Putin groups to the series of demonstrations held in recent months by the political opposition, drew 140,000 attendees, according to police estimates. Some independent observers put attendance as low as 25,000.
But the application to hold the event specified that only up to 15,000 people would attend.
A police officer testified at a hearing Friday that it became obvious to him only an hour after the event began that it would draw more than 15,000 participants.
Rally organizer and Patriots of Russia party member Nadezhda Korneyeva admitted her guilt and agreed to pay the fine, RIA-Novosti reported.
After it was reported Saturday that organizers may be forced to pay a fine for having too many people at the event, Prime Minister Vladimir Putin offered to pay it.
Organizers of the opposition march and rally that took place on Saturday may also face a fine for having too many people at their event.