The fire broke out around 9:30 p.m.at a house on Montreal's South Shore, rented as the election headquarters for Guy Brault, leader of Parti des citoyens – Équipe Guy Brault.
The Sûreté du Québec said the house is not destroyed, but the damage is extensive.
Brault said his team lost electronic equipment, telephones and his party’s voter call lists, which burned in the fire.
Last Thursday, Brault said a Molotov cocktail was thrown through the office window, causing minor damage to the floor.
"I think there are people who see the advance polls and that bothers some people. You have the proof tonight," said Mario Arsenault, councillor and member of Brault's team.
Brault is trying to unseat the incumbent mayor, Gilles Pepin.