The riverside town of Ercsi, on the boundary between the Soviet 23rd Rifle Corps and 3 7th Rifle Corps sectors, would form the base of supply for both formations plus the 2nd Guards Mechanized Corps expected to exploit from there to the northwest.

Along the riverbank, the defenders had spent days reinforcing their positions. Between eight and ten firing pits and two or three machine-gun positions supported every 100 meters of trench line, along with small earthen bunkers. However, no other field fortifications had been constructed behind the river trench line. Upon breaching the Axis main line of resistance, the Soviets would be able to move inland easily. At 2300 on 4 December, the first assault groups began crossing through the convenient fog without any artillery preparation.

The 271st People's Grenadier Division met the crossing with a wall of artillery and machine-gun fire. At 2340, the 46th Army answered with an intense artillery barrage directed at the newly revealed Axis firing positions. Axis fire destroyed many Soviet boats in mid-river but eventually most of the crossing troops made it to the opposite bank, either undisturbed or with minimal loss. South of Ercsi, the initial assault secured the shoreline during the first hours of assault, so that 59th Guards Rifle Division could cross almost without loss.

Ercsi itself lay in the sector of the 108th Guards Rifle Division. The fighting in the city lasted for a long time; one People's Grenadier battalion even fought until 6 December in some parts of the city. Further north, a German counter-attack repulsed the landing by a 99th Rifle Division battalion. Even further north, the 1077th Rifle Regiment of the 316th Rifle Division crossed with fewer losses but had to fight the whole day just to secure its own slender bridgehead. It took the 46th Army two to three hours (instead of the planned one to one and half) to transfer the first assault wave across.

At 0200, the Soviets managed to get into the first Axis trench line but only secured it after two more hours of fighting. Heavy street fighting continued through the rest of the night and the entire morning, completely disrupting the Soviet timetable. At dawn, the army's chemical troops laid smokescreens to protect the ferry crossing and by 1100 the 108th Guards Rifle Division finally seized Ercsi after fierce street battle.