Does the erection of the penis or the arousal of desire during prayer invalidate it, and what should be done about that?

Prayer is not invalidated merely by an erection or the arousal of desire, as long as no madhy (pre-seminal fluid) is discharged. If madhy is discharged, then ablution is nullified, and it is obligatory to exit the prayer, wash the penis and whatever madhy has touched on the clothes or body, and repeat the prayer. One should ward off the whispers and thoughts that cause this by seeking refuge with Allah from Satan and spitting lightly to the left three times.