Ahmed Dogan, head of the Movement for Rights and Freedoms party, was speaking to delegates at a party conference at the National Palace of Culture in Sofia when the gunman came up to the podium and extended his gun toward Dogan, but failed to get a shot off.
A video shows Dogan appearing to knock his assailant's hand away before the two men fell to the floor. The would-be assassin was overpowered by security personnel and beaten and kicked by party delegates, The Sofia Globe reported.
Tim Harris, a South African member of Parliament who was in the hall along with visitors from various countries, said the assailant pulled the trigger twice.
The bloodied assailant, who also was found to be carrying a knife, was taken to a nearby police station, the newspaper said.
Interior Minister Tsvetan Tsvetanov identified the suspect as Oktai Enimehmedov, a 25-year-old Bulgarian national and ethnic Turk from Bourgas. The ministry said he has prior convictions in 2006 for causing bodily harm and 2010 for drug possession.
Tsvetanov said the suspect had an accreditation badge from a previous party event and got the podium without being stopped. Police were trying to determine whether he was a party member.
Dogan was one of the party's founders and has been its leader since 1989. He has been described as a kingmaker in Bulgarian politics, with his party providing key support to prime ministers Simeon Saxe-Coburg and Sergei Stanishev.
The Globe said there had been reports in the Bulgarian media that Dogan planned to step down in favor of long-time deputy Lyutfi Mestan, and said it had established his prepared speech text included the announcement that he intended not to continue as leader of the party.