Guards whisked Pauline Marois off the stage as handlers informed the partisan crowd there had been an explosive noise and they needed to clear the auditorium.
Police tackled a housecoat-wearing suspect to the ground, and took him away in a patrol car. Local authorities said two people were shot and were in critical condition. They later announced one person had died.
The scantily clad suspect, a man in his 50s, shouted in French at the TV cameras while being dragged toward the police cruiser: "The English are waking up!"
A PQ spokesman asked the crowd to leave when Marois was taken from the stage and the crowd milled around, confused.
The premier-elect soon reappeared and finished her speech but security hovered nearby, with one man talking constantly into a wrist microphone.
The crowd cleared out afterward, passing by Montreal police who lined the lobby and appeared at the back of the hall. Marois was escorted from the building in a tight cordon of provincial police bodyguards.
A fire truck was parked on the street and firemen entered as police put up orange crime scene tape across one corner of the stage leading to an exit.
An acrid smell of smoke hung heavily in the bar as media collected their equipment while other police officers moved around the stage and talked to the hall's own security guards.
The attack occurred at the Metropolis, not far from the busy intersection of Saint-Laurent and Sainte-Catherine. The club is one of the best-known music venues in the city and regularly plays host to popular groups.
Montreal police spokesman Danny Richer originally said the man was able to enter the building but he was not sure how close he got to the new premier.
Police later said it was unclear whether he got into the building.
Police did not confirm what kind of weapon was used, but TV images appeared to show a long gun being confiscated.
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