The commemorative service is set to begin at the Notre-Dame Basilica at 3 p.m.
Canadian Prime Minister Stephen Harper is expected to deliver an address.
Desmarais was a mentor to several Canadian leaders, including Brian Mulroney and Jean Chrétien, both of whom will be attending the service.
The guest list also includes former French president Nicolas Sarkozy, former U.S. secretary of state James Baker, Quebec Premier Pauline Marois and Canadian Gov. Gen. David Johnston.
The self-made billionaire died in October surrounded by family at Domaine Laforest, the family's sprawling estate in Quebec's Charlevoix region.
He was 86.
Desmarais, whose political and business connections reached across Canada and overseas, started out with his family's bus firm in Sudbury.
He went on to build international conglomerate Power Corp. and gain an estimated personal net worth of around $4.4 billion.