Castro, who ceded control to his younger brother Raul Castro ten years ago due to poor health, died on Friday at the age of 90, leaving behind a mixed legacy.
For many, especially in Latin America and Africa, he was a symbol of resistance to imperialism, having ousted a US-backed dictator, and a champion of the poor.
Others condemned him as a tyrant whose socialism ran the economy to ruin.
Cuba announced nine days of mourning after his death, including the mass rally this evening in Revolution Square – the same massive space where Castro once held fiery, marathon speeches.
Many leaders of Latin America”s left, including Venezuelan President Nicolas Maduro and Bolivian President Evo Morales, were to attend the ceremony.
Shortly after landing in Havana last night, President Maduro paid tribute to Castro’s “immortal force”.
Also expected are several African leaders such as Zimbabwe”s Robert Mugabe and South Africa”s Jacob Zuma.
Nelson Mandela, while he was still alive, repeatedly thanked Castro for his efforts in helping to weaken apartheid in South Africa.
Yet few leaders from the world”s major powers are heading to the Caribbean island, with many sending second tier officials instead to pay their respects to a man who built a Communist state on the doorstep of the United States.
All schools and government offices will be closed on Tuesday so that Cubans can more easily join the rally and other activities to pay homage to Castro, authorities said.
This morning, they will have their last chance to pass by a portrait of the late leader, dressed in military fatigues and carrying a rifle, erected in a memorial to Cuban independence hero Jose Marti in Revolution Square.
Tens of thousands already did so yesterday, some in tears and others wrapped in the red, white and blue national flag.
Many state employees and school children came together in groups.
Raul Castro and his top lieutenants held a separate, private ceremony yesterday, laying white flowers in front of Fidel Castro”s portrait.
Cubans have also been urged to sign condolence books and pledges of loyalty to Castro”s socialist ideology at 1,060 tribute sites throughout the country.
“I signed because he was a good man, we loved him a lot, and I wanted to reaffirm my loyalty to him and his ideas,” said Arcide Ge, 56, a security guard. “He was good to everyone, he sent doctors abroad and helped the poor here.”
Tomorrow, Castro”s ashes will begin a procession east across the country towards Santiago de Cuba, where he launched the revolution.
They will be laid to rest on Sunday December 4, in the city”s Santa Ifigenia cemetery, also the resting place of Marti.