Petar Ovčarević hailed from Belgrade, and was a "hajduk". As a youngster he was captured by the Ottomans, but managed to have himself freed and join the Ottoman army. He for long served the Ottomans, then joined Hungarian service. He was a commander of the "Šajkaši" (Danube river flotilla) in Zemun, and during the Siege of Belgrade (1521) was the main commander of the Šajkaši in Belgrade. After surviving the siege, which ended in Ottoman takeover, and vacillating for a time, he joined the Ottomans and became a confidant of Ottoman general and sanjak-bey of Belgrade, Bali Bey Jahjapašić. He became a "Martolos" commander, commanding Serbs, based by the Belgrade rivers, inhabiting their own quarter and commanding a galley fleet. His unit was entrusted with the protection of Ottoman ferries on the Sava and Danube.