The fighting continued and on August 4 the besiegers began to bomb the castles with obuses, repeating this operation every night until the 14th in order to keep the royalist soldiers fatigued and demoralized.

Meanwhile, from Cuzco, Viceroy La Serna had planned an incursion on the Lima valley, which, led by General José de Canterac, commanding 3,100 soldiers and 9 artillery pieces, left Jauja at the end of August.

After crossing the Andean mountain range in a difficult journey and having to face the montoneros, the troops reached the outskirts of the capital on September 8, where they found the patriot army maintaining defensive positions.

Although San Martín had a superior army based on the number of troops, he decided not to attack despite the insistence of General Las Heras, thus allowing Canterac to march until he entered Callao.

As the possibilities of the besieged receiving reinforcements or supplies of any kind became remote, San Martín offered La Mar an honorable surrender, assuring him that Canterac's army was moving away in a disastrous state and in frank dispersion.

The conditions of capitulation allowed the royalist troops to withdraw in full dress, carrying their light weapons and with their flags unfurled, being saluted in the march by the artillery of the patriot army.