On the evening of August 20, its commander, Colonel Stefan Dąb-Biernacki, ordered his soldiers to capture Białystok, together with its railroad junction, and to cut off the enemy's roads of retreat.

On August 21, after a 40 km march, the vanguard companies of the regiment captured the village of Zwierki, 14 kilometers from Białystok.

They captured 100 prisoners and nine machine guns, but some Soviet soldiers managed to escape and warned the garrison of Białystok of the Polish raid.

Despite warnings, the defenders of Białystok were surprised by the 3rd Battalion of the 1st Legions' Regiment, under Captain Jozef Marski-Marjanski, which broke into the centre of the city.

The 164th Rifle Brigade was completely destroyed, with 2,000 prisoners taken, together with 13 cannons, 30 machine guns and three military trains with food and equipment.

Polish forces did not have time to celebrate their victory, as at 8:15 a.m., the first units of Soviet 16th Army approached the city.

Enjoying numerical superiority, the Red Army seized the main rail station and entered the centre of Białystok.