The shootout occurred outside San Lorenzo's El Nuevo Gasometro stadium, where Boca were scheduled to play the hosts.
Hours before kick-off, several shots were fired and two people killed, leading to the suspension of the friendly.
San Lorenzo confirmed the deaths on their website, saying the incident had occurred at 2.50pm local time.
Violence is a major problem in South American football, particularly in Argentina where a Lanus fan was killed in June.
That incident happened during fighting between hooligans and riot police before Lanus' match against Estudiantes.comments