The place where Castro would have watched the festivities — a raised platform under a towering statue of Cuban colonial independence hero Jose Marti — was instead occupied by his brother Raul.
Castro had attended the annual International Workers' Day march for decades. But the 80-year-old communist leader has not been seen in public since emergency intestinal surgery forced him to step down temporarily nine months ago and temporarily cede his duties to his 75-year-old brother.
In a speech just before the huge annual parade began, Salvador Valdes, secretary general of Cuba's central workers union, hinted it was highly unlikely that Castro would make his first public appearance since stepping down from power.
"A speedy recovery and lots of health, dear Fidel," Valdes said, adding "Viva Fidel!" as the crowd shouted back "Viva!"
The acting president and defense minister, five years younger than his sibling, stood stiffly and smiled, occasionally waving as marchers streamed past, waving plastic Cuban flags, portraits of Fidel Castro and banners denouncing U.S. "imperialism."
Many marchers wore red or white T-shirts that appeared to have been distributed by the Cuban government with a May 1 slogan reading "More solidarity, unity and strength than ever."
Castro issued the latest in a new series of communiques Monday, calling for a revolution in energy production, but gave no hint of whether he would make a May Day appearance.
He reiterated his opposition to U.S. plans to use food crops to produce ethanol for cars, predicting that American fuel needs would be provided with the labor of the world's impoverished sugar cane workers.
"Tomorrow the first of May is a good day to carry these reflections to the workers and all of the poor people of the world," Castro wrote in the communique.
Occasional government photographs and videos of the elder Castro have assured Cubans he is still alive and recovering, appearing stronger and more robust in the most recent images.
But the urgent need by many to see the man who ruled this country for 47 straight years seems to have faded in recent months as life has continued normally under the leadership of Raul Castro.
There will be smaller marches Monday in cities around the island, with the government expecting several million of the nation's 11 million people to participate.
Those gathered in Havana protested the recent U.S. court decision to free on bond anti-communist militant Luis Posada Carriles, pending his trial on U.S. immigration charges. Havana accuses the Cuban-born Posada of orchestrating a 1976 airliner bombing that killed 73 people — a charge he denies.
Demonstrators held signs and banners declaring "Prison for the Executioner" in reference to Posada, accusing the Bush administration of a double standard on terrorism.
Click here for FOXNews.com's Americas center.