A man pointed what turned out to be a gas pistol at the leader of the ethnic Turks' Movement for Rights and Freedoms, Ahmed Dogan, during a party conference on Saturday without injuring him.
The reasons for the assault remain unclear.
The attacker, who did not fire a shot, was quickly disarmed by delegates and severely beaten, a television footage aired by the Bulgarian National Television showed.
The police later said in a statement that the 25-year-old attacker, a Bulgarian citizen and member of the Turkish minority, had two knives on him, as well. He had been convicted of causing bodily harm in 2006 and drug possession in 2010, the interior ministry said.
The assault was condemned by politicians from all local parties.
Following the attack, the party conference unanimously elected Lytvi Mestan as the new party leader. Dogan remains its lifetime honorary chairman.
Dogan, 58, has led the party since its formation 23 years ago. The movement was one of three parties which formed a coalition for the previous government, along with the Socialists and the National Movement for Surge and Stability led by ex-monarch Simeon Saxe Coburg-Gotha.