After a subdued weekend following his death on Friday, hundreds of thousands are expected to flock to the plaza where Castro would often rail against the US "yankees" and "empire" during his legendary, marathon speeches.
"You'll see how the people of Cuba really are. You'll see how they are suffering, how they feel about a person they love," said Jorge Guilarte, a 50-year-old bike-taxi driver.
Castro, whose 1959 revolution toppled a dictatorship with the promise of bringing justice and equality to his Caribbean island, was a major 20th century figure.
In a sign of changing times, US President Barack Obama visited the plaza during his historic visit to Havana in March, when he became the first US leader since 1928 to step foot in Cuba.
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In 2014, Fidel's brother and successor, Raul Castro, announced a diplomatic detente with Obama, who has lifted some trade barriers. On Monday, the first regular flights from the United States to the Cuban capital will resume.
Fidel handed power to Raul Castro in 2006 after undergoing emergency intestinal surgery. His cause of death on Friday at age 90 has not been disclosed.
At the Revolution Square, famous for a government building adorned with the face of Argentine-born guerrilla Ernesto "Che" Guevara, organisers installed a giant photo at the National Library of the Fidel Castro carrying a rifle during the revolution that brought him to power.
Officials have yet to confirm whether an urn carrying his ashes will be placed on a platform so that Cubans can file in front of his remains.
"Saying goodbye to Fidel will be beautiful," she said.
Dissidents who were repressed by his regime for years said they were happy that the "dictator" had died, but they called off regular demonstrations yesterday out of deference to those in mourning.
"We are not happy about the death of a man, a human being. We are happy about the death of dictators," Berta Soler, leader of the Ladies in White, told AFP.