"This is a fraud and a slap in the face of the Croatian people," Miksic told between 1,500 and 2,000 supporters who gathered in the city's central square.
The protesters proceeded from the square to the building housing the offices of the State Election Commission, shouting "No more fraud", "Let's go all the way" and "No surrender" and demanding new elections.
Addressing the protesters, Miksic said they wanted nothing but justice and dignity.
Headed by Miksic, the protesters walked by the parliament building, shouting "We don't want thieves" and singing patriotic songs. After that, Miksic called on his supporters to disperse peacefully.
Followed by shouts "Fight, Boris", Miksic left the rally in his car, while several hundred of his supporters proceeded to the building housing the offices of the Croatian Democratic Union (HDZ), where they continued the protest.
The protesters were also addressed by the editor of the Novo Hrvatsko Slovo weekly, Domagoj Margetic, who levelled grave accusations at the HDZ. The presidential candidate of the Croatian Youth Party, Miroslav Rajh, expressed support for Miksic.
At the time of the protest, the HDZ's National Committee was in session.
Miksic's supporters in the eastern city of Osijek this evening started signing a petition of support, which was signed by 400 people in the first 30 minutes.
Some 300 sympathisers gathered in Split, southern Croatia, marching through the city carrying a banner saying "Fight, Boris".
Around 200 people gathered for a rally in Karlovac, some 50 kilometres south-west of Zagreb, to sign a petition demanding that votes be recounted.