Hundreds of thousands of people packed the capital's Revolution Square, chanting "I am Fidel!" across the vast esplanade where Castro gave many of his legendary, marathon speeches.
A giant picture of a young, bearded Castro in his guerrilla uniform and rifle hung on the National Library as his brother and successor, Raul Castro, waved at the crowd.
It was the end of two days of tributes in Havana, before Fidel's ashes are taken on a four-day-long procession across the country on Wednesday.
One after the other, Latin American, African and Caribbean leaders - along with Greece's prime minister, the only European leader at the event - lionized the communist leader.
"Mission accomplished, comandante Fidel Castro," said Venezuelan President Nicolas Maduro, whose late mentor, Hugo Chavez, had a special bond with Castro.
"Today it is up to us to raise the flags of independence of the great fatherland, today it is up to us to hold the flag of dignity and freedom of the people."
Maduro declared that Castro was "totally absolved by history," in reference to the Cuban leader's famous phrase "history will absolve me" at a trial following a failed uprising in 1953.
Ecuador's leftist President Rafael Correa praised Castro's ideology, telling the crowd: "We will keep fighting for these ideas. We swear!"
Leaders praised Castro in almost religious tones, with Bolivian President Evo Morales saying, "Fidel is not dead ... Fidel is more alive than ever, more necessary than ever."
Raul Castro, 85, spoke last at the rally, thanking the "countless gestures of solidarity and affection from around the world" and ending with the revolutionary battle cry, "Until victory, always!"
Castro's death, however, comes as Latin America's left is losing ground.
Maduro is facing a deep economic crisis and fighting opposition attempts to hold a recall referendum, while Brazil's Dilma Rousseff was impeached in August and a conservative took over in Argentina last year.
Castro - who ruled from 1959 until an illness forced him to hand power to his brother Raul in 2006 -- died Friday at age 90.
South African President Jacob Zuma hailed Castro as "one of the great heroes of the 20th century," citing his opposition to apartheid and his deployment of Cuban troops to back Angola's government against rebels in 1975.
Washington's 'significant concerns'
But several world leaders shunned the tribute, highlighting the divisive legacy of the major Cold War player.
The leaders of Britain, Germany, France, Spain and Canada dispatched others in their place, but even the presidents of friendly nations such as Russia, China and Iran sent deputies.
Castro spent decades feuding with the United States, brought the world to the brink of nuclear war during the 1962 missile crisis, backed guerrilla movements in Latin America and deployed his army to conflicts in Africa.
US President Barack Obama, who along with Raul Castro ended decades of enmity to restore diplomatic relations, did not attend. A senior advisor and the top US diplomat in Cuba were designated, but without the status of a "presidential delegation."
"We continue to have some significant concerns about the way the Cuban government currently operates, particularly with regard to protecting the basic human rights of the Cuban people," White House spokesman Josh Earnest said.
'Caravan of freedom'
The rally followed two days during which Cubans, encouraged by the government, streamed past a picture of Castro inside the square's towering monument to 19th century independence hero Jose Marti.
"Fidel would be proud to see the square overflowing like this, especially with young people," said 46-year-old teacher Tatiana Gonzalez.
Cubans were also urged to sign an oath of loyalty to Castro's revolution in books placed at schools and other public buildings.
After Tuesday's ceremony, the urn holding Castro's ashes will be taken on a "caravan of freedom" across the country, retracing the route his guerrilla movement took to celebrate the toppling of dictator Fulgencio Batista in 1959.
The commemorations end Sunday, when the urn is laid to rest in the eastern city of Santiago de Cuba, where Marti is buried.