The Bulgarian Emperor deduced that his opponent would go through the Tryavna Pass. The Byzantine army slowly marched southwards, their troops and baggage train stretching for kilometers. The Bulgarians reached the pass before them and staged an ambush from the heights of a narrow gorge. The Byzantine vanguard concentrated their attack on the centre where the Bulgarian leaders were positioned, but once the two main forces met and hand-to-hand combat ensued, the Bulgarians stationed on the heights showered the Byzantine force below with rocks and arrows. In panic, the Byzantines broke up and began a disorganized retreat, prompting a Bulgarian charge, who slaughtered everyone on their way. Isaac II barely escaped; his guards had to cut a path through their own soldiers, enabling their commander's flight from the rout. The Byzantine historian Niketas Choniates wrote that only Isaac Angelos escaped and most of the others perished.

The battle was a major catastrophe for the Byzantines. The victorious army captured the imperial treasure including the golden helmet of the Byzantine Emperors, the crown and the Imperial Cross which was considered the most valuable possession of the Byzantine rulers - a solid gold reliquary containing a piece of the Holy Cross. It was thrown in the river by a Byzantine cleric but was recovered by the Bulgarians. The victory was very important for Bulgaria. Up to that moment, the official Emperor was Peter IV, but, after the major successes of his younger brother, he was proclaimed Emperor later that year.