Later footage showed officers carefully retrieving a rifle from behind the hall as a handcuffed man lay nearby. The man, who was wearing a bathrobe, black underwear and a black balaclava, was later placed in a police cruiser. He used a French expression that can be translated as, “The English are awakening” or “The English are rising up.”

Police officials said they knew little about the man except that he was 62; earlier they had said he was about 50. His motives and intention were not immediately clear.

Ms. Marois later reappeared on the stage and said there had “been an incident with a starter pistol or something” and urged her supporters to leave quietly.

Many observers characterized the Parti Québécois as more of a rejection of Quebec’s scandal-plagued Liberal Party than an endorsement of separation. Ms. Marois’s party captured the largest number of seats — 54 — in the 125-seat provincial assembly in Tuesday’s election, fewer than the number needed for a majority.

For several years, polls have suggested that there is relatively little interest among Quebec residents in separating from Canada. But breaking with recent Parti Québécois tradition, Ms. Marois made it an important issue of her campaign.