Pauline Marois, who was elected as Quebec's first female premier, had just begun to speak to her supporters when her bodyguards whisked her away from the stage. She returned later to finish her speech. The incident was shocking for Canada, a country known for its peaceful culture and which had seen its last political killing in 1970.
Police sources said the assassin, aged around 50, had entered the back of the theatre just before midnight and shot two people.
The deceased was in his 40s while the other person was taken to hospital in a critical condition. The assassin appeared to shout in French: "The English are waking up", Reuters said. It is learnt that Marois's plan to strengthen laws to ensure dominance of French has left the minority English-speaking sections a worried lot and the attack could have been a consequence of that.
Federal government sources said they were appalled by the violence. Montreal police had reportedly cordoned off a truck which they had suspected of containing weapons. Other sources said the deceased was a technician at the theatre while the injured man was a driver of the bus used for PQ campaign, Reuters said.
The PQ had won 54 of the 125 seats in the provincial legislature, ending the Liberals' nine years of rule.
(With agency inputs)