Police announced midday Wednesday the suspect arrested at the scene was Richard Bain, 61, the Canadian Broadcasting Corp. reported.
Just before midnight, as newly elected separatist Parti Quebecois leader Pauline Marois, the next premier, was giving a victory speech at the Metropolis Theater, a gunman opened fire outside the back of the building.
One man was killed and another seriously injured. The gunman then started a large fire at the door that had been slammed shut and locked, police said.
Television footage showed the suspect yelling "the Anglos are waking up" in French with an English accent as police led him away.
Police sources told the Globe and Mail an assault weapon and a handgun were seized at the scene and a canister of flammable material was found in the suspect's vehicle.
Inside, Marois was hustled off the stage by security staff. When police gave the all-clear, she returned to finish her speech.
Canadian Prime Minister Stephen Harper issued a statement Wednesday condemning the violence.
"I was angered and saddened to hear of last night's horrific shooting at the Parti Quebecois event at Metropolis," Harper said. "This atrocious act will not be tolerated and such violence has no place in Canada."
The Parti Quebecois unseated the three-term Liberals and has a mandate of separating from Canada.