The fight between Other Russia member Andrei Pesotsky and former member Igor Chepkasovy was a continuation of a feud the pair had been having online.
During the encounter, which took place at a discussion of a planned For Fair Elections march, Pesotsky fired at his adversary from a gas pistol, a non-lethal weapon that can be used to fire flares or as a movie prop.
Upon hearing the shot, a third activist, nationalist Nikolai Bondarik, approached and fired a blank into the air from his own starter pistol.
No one was injured in the incident.
Meetings held in recent months by opposition leaders to organize public rallies have been marked by frequent strife, but this appeared to be the first time shots had been fired at such an event.