The video from the Saturday event in Sofia shows the man climbing the podium where Ahmed Dogan, the leader of the Movement for Rights and Freedoms, was speaking, and pointing the gun to his face.
Dogan struck the man before he could pull the trigger, while other delegates wrestled the assailant to the ground. TV footage showed several people punching, kicking and stomping on the man when he was on the ground.
Police arrested him and took him to a hospital. It wasn't immediately clear if he sustained serious injuries, or how he got past security to enter the hall with nearly 3,000 people attending.
Eventually, the attacker was identified by police as 25-year-old Oktai Enimehmedov, a Bulgarian national and ethnic Turk, from the coastal city of Burgas. He was carrying the gas pistol and two knives. A gas pistol is a non-lethal weapon used for self-defense, but experts say when fired from close range it can cause life-threatening injuries.
Interior Minister Tsvevtan Tsvetanov told journalists that the assailant had a criminal record for drugs possession, robberies and hooliganism.
The liberal MRF party mainly represents ethnic Turks and other Muslims in Bulgaria, who make up 12 percent of its 7.3-million population.
The conference had to elect a new leader to succeed Dogan, who is one of the Balkan country's most influential political figures. The 58-year-old has been at the helm of the party since founding it in 1990.
Lyutvi Mestan, who was expected to become the new party leader, said "the true reason for the assault was the language of hatred and confrontation."
Saturday's assault was the gravest attack on a politician in post-communist Bulgaria after the 1996 killing of ex-Prime Minister Andrei Lukanov.
The German government's human rights commissioner, Markus Loening, was at the party caucus in Sofia on Saturday, and witnessed the incident, the German Foreign Ministry said. "I am very happy that Ahmed Dogan was not injured," Loening said in a statement. "Political violence must never again gain a foothold in Europe."