Three women pulled alive from the mangled wreckage are the only known survivors.
The Boeing crashed shortly after taking off from Jose Marti airport, coming down in a field near the airport and sending a thick column of acrid smoke into the air.
The mourning period is to last from dawn on Saturday to midnight on Sunday, the Communist Party leader and former president Raul Castro said. Flags are to be flown at half-mast throughout the country.
The plane was on an internal flight from Havana to the eastern city of Holguin. Most of the passengers were Cuban, with five foreigners, including two Argentines, among them.
The plane — carrying 104 passengers — was almost completely destroyed in the crash and subsequent fire. Firefighters raced to the scene to put out the blaze along with a fleet of ambulances to assist any survivors.
What appeared to be one of the wings of the plane was wedged among scorched tree trunks, but the main fuselage was almost entirely destroyed.
Built in 1979, the plane was leased from a small Mexican company, Global Air, also known as Aerolineas Damoj.
Mexico said it was sending two civil aviation specialists to help in the investigation. The six crew members were Mexican nationals.
Boeing issued a statement saying that a “technical team stands ready to assist” and offering condolences to friends and relatives of the victims.