While the victorious Bulgarian army was marching southwards, the Byzantine commander Leo Phokas, who survived at Achelous, reached Constantinople by sea and gathered the last Byzantine troops to intercept his enemy before reaching the capital. The two armies clashed near the village of Katasyrtai just outside the city and after a night fight, the Byzantines were completely routed from the battlefield.

The last Byzantine military forces were literally destroyed and the way to Constantinople was opened, but the Serbs rebelled to the west and the Bulgarians decided to secure their rear before the final assault of the Byzantine capital which gave the enemy precious time to recover.