A man jumped on stage and pointed a gas pistol at the leader of Bulgaria's ethnic Turkish party before security guards wrestled him to the ground during a live televised conference.
Ahmed Dogan, the long-time leader of the Movement for Rights and Freedoms (MRF) escaped unscathed - and it was not immediately clear why he had been targeted at the party congress in Sofia.
Television footage showed a man jumping out of the audience and interrupting a speech by the 58-year-old Mr Dogan, who has led the party for almost a quarter of a century.
Security guards were seen beating and kicking the man, who was also carrying two knives, after he pointed the non-lethal weapon at Mr Dogan's head.
"Ahmed Dogan is in good health. Everything is under control," MRF official Ceyhan Ibryamov told journalists.
Police said they had arrested a 25-year-old man from the Black Sea town of Burgas. They said the attacker was also carrying two knives.
The liberal MRF party represents ethnic Turks and other Muslims, who make up about 12% of Bulgaria's 7.3 million population.
Mr Dogan is seen as one of Balkan country's most influential political figures. The MRF was a junior partner in the previous
He tendered his resignation hours after the attack, saying: "This time my decision is categorical."
He was widely expected to step down from his position, even before the attack.
In 1996, former Prime Minister Andrei Lukanov was found shot dead near his home in Sofia - although attacks on politicians are actually rare.