On 20 September two Polish regiments (commanded by Władysław Bończa-Uzdowski and Wacław Scaevola-Wieczorkiewicz) began their assault on the 11th Soviet Rifle Division.

In the morning of 21 September, 6th Legions Brigade surprised Soviet 16th Rifle Division, and captured all villages, except for Brzostowica Mala.

Fighting continued until the evening, when General Kork ordered Soviet 56th Rifle Brigade (3000 soldiers, 54 heavy machine guns) to join the forces near Brzostowica.

The well-rested and well-equipped brigade was placed on the right wing of Soviet forces, as Kork wanted to regain the positions along the Swislocz river.

Near the road toward Wolkowysk, Soviet 56th Rifle Brigade entered the fray, attacking Polish 7th Legions Infantry Regiment.

At 6 p.m., after artillery barrage, Soviet 33rd Rifle Brigade attacked the Poles, but due to concentrated Polish fire, it suffered heavy losses and returned to the starting positions.

Soon afterwards, Polish lines were broken near Brzostowica Mala, where 9th Legions Infantry Regiment, attacked by Soviet forces, retreated five kilometers back, towards Swislocz.

In a desperate attempt, Boncza-Uzdowski threw all his forces into the fray, and by midnight, Poles reached the line Masalany - Sedejki - Werejki.