The rebels first clashed with Ottoman regulars at the Battle of Ivankovac in August 1805, scoring a decisive victory. Belgrade was soon besieged by about 16,000 rebel fighters. The rebel leaders used anti-Muslim rhetoric to mobilize the peasantry, calling upon them to rise up and drive the Ottomans "across the blue sea". In order to further galvanize the population, Karađorđe appealed to memories of the Battle of Kosovo of June 1389, which paved the way for the Ottoman conquest of Serbia and the rest of the western Balkans, declaring that Serbia's defeat in the battle needed to be avenged. He sought to further cement his authority by harkening back to symbols of Medieval Serbia, such as the relics of Stefan the First-Crowned, and placing old heraldic symbols on flags and seals to establish continuity between the Serbian Empire and himself. Portraits of Dušan the Mighty, the founder of the Serbian Empire, are said to have hung from the walls of the rebels' headquarters.