Ahmed Dogan, 58, who is the leader of the Movement for Rights and Freedoms party, struck the attacker before he was able to fire any shots.
A gas pistol is a non-lethal weapon used for self-defense, however when fired at close-range, it can cause life-threatening injuries.
Video of the skirmish shows other delegates wrestling the gunman to the ground, and then kicking and beating him.
The suspect was identified by police as 25-year-old Oktai Enimehmedov, who is an ethnic Turk, according to The Associated Press.
He was arrested and taken to a local hospital, but it was not known what injuries he may have sustained in the scuffle.
The MRF party represents mostly ethnic Turks and Muslims in Bulgaria, however it was unclear what the gunman’s motive was for the attack.
The attack was the worst on a Bulgarian politician since the 1996 slaying of ex-prime minister Andrei Lukanov.