About 2,000 sympathisers of Miksic gathered in Zagreb's central Ban Jelacic Square on Monday evening saying that Sunday's presidential election was rigged and that their candidate had been robbed of votes.
Under the law, any gathering attended by more than 50 people must be reported to the police.
Miksic had called for a peaceful rally via the media, but he and his election team did not report it to the police, Zagreb Police Department spokeswoman Gordana Vulama said at a press conference.
Vulama said that Miksic faced a fine of between 5,000 and 20,000 kuna (between approximately 660 and 2,600 euros) for this violation.
No incidents were reported during the protest rally.