One person was killed and another was wounded in the attack, which occurred as Parti Quebecois leader Pauline Marois was giving her victory speech after her party won Tuesday's provincial elections.
Police said suspect Richard Henry Bain, 62, was detained outside the hall, where he shouted in French as he was being taken away: "The English are waking up!"
The province, which has a French-speaking majority, has a long history of contentious language politics. At the rally, Marois said in English that she would protect the rights of Anglophones.
At the sound of the gunfire, Marois was whisked off the stage by guards. Police said they don't know whether Marois, who will be Quebec's first female premier, was the intended target. Both of the shooting victims were members of technical crew at the event, police said. The person who was wounded is hospitalized in critical condition.
Police said the alleged gunman, who was wearing a robe and a balaclava -- a type of ski mask -- also set a fire behind the hall where the rally was taking place. An assault rifle and a handgun were recovered at the scene.
While winning the most seats in the election, Marois' party did not win an outright majority and will have to work with other parties to retain power, thus making it difficult to push a separatist agenda. A recent poll showed support for Quebec independence decreased during the provincial campaign.
-- Kristina Jovanovski
Photo: Police arrest a suspected gunman outside the Parti Quebecois victory rally in Montreal early Wednesday. Credit: Olivier Pontbriand / Associated Press / Montreal La Presse via The Canadian Press