This is the first time in 40 years that Castro hasn't made it to the May Day parade, a celebration that would have been unthinkable without him a year ago, reports CBS News senior foreign correspondent Lara Logan.
While recent images of Castro meeting with Chinese leaders indicated he had improved considerably since undergoing emergency surgery nine months ago, his absence at the parade through the Revolution Plaza raised questions about whether he is strong enough to run the country.
The 80-year-old leader has missed two other major events since announcing his illness on July 31 and temporarily ceding power to his 75-year-old brother Raul Castro, the defense minister. Raul presided at the Nonaligned Summit in September and a major military parade in December.
"It now seems more unlikely than before that he will fully resume the presidency," said Wayne Smith, the former head of the American mission in Havana. "And the more time that passes, the more unlikely it seems."
Smith said that with Castro failing to show Tuesday, Venezuelan President Hugo Chavez's assertions this week that Castro was back "in charge" appeared to be "a lot of hot air."
Others said he could still resume some responsibility.
"To me, the key question is to what degree is he coming back?" said Phil Peters, a Cuba specialist for the Lexington Institute, a Washington-area think tank. "Would his comeback be partial, ceremonial? Will he spend two hours in the office checking off on strategic decisions?"
Raul Castro, wearing his typical olive-green uniform and cap, stood stiffly and smiled under the shadow of a towering statue of Cuban independence leader Jose Marti. He occasionally waved as marchers clad in red T-shirts and dark slacks streamed past, clutching plastic Cuban flags, portraits of his more famous brother and banners denouncing U.S. "imperialism."
Fidel Castro's absence is hardly a shock to a nation that already seems to be getting by without him, reports Logan.
Although Cuban life is little changed under Raul's leadership, loyalists missed the energy Fidel brought to events such as May Day.
"Everyone wanted to see him, but it's good that he recovers completely. Now the revolution is continuing with Raul," said 68-year-old hotel worker Victor Reyes, who was among the marchers.
Special guests included a Cuba solidarity group from New York. The foreigners were impressed by the large turnout, which Havana's Radio Reloj estimated at 500,000. Smaller marches were held simultaneously in cities around the island.
"Even without (Fidel Castro), they came out en masse," said Joppe Van Meervelde, 29, a metal workers' unionist from Belgium.
Marchers protested the recentLuis Posada Carriles, pending his trial on U.S. immigration charges. Havana accuses the Cuba-born Posada of orchestrating a 1976 airliner bombing that killed 73 people — a charge he denies.
Signs and banners demanded "Prison for the Executioner" and accused the U.S. government of a double standard on terrorism in the Posada case.
Marchers also demanded the release of five Cuban spies imprisoned in the U.S. for being unregistered foreign agents, calling them heroes who were merely protecting their country from violent exile groups.
In the hours before the march began, Fidel Castro issued the latest in a series of communiqués — but gave no hint he would show up in public.
CBS News producer Portia Siegelbaum reports from Havana that the 10-page statement, entitled "Reflections of the Commander in Chief", is a call for "an energy revolution". Fidel Castro often focuses his attention publicly on protecting the world's poor from plunder by developed countries.
Castro reiterated his opposition to U.S. plans to use food crops to produce ethanol for cars, predicting that American fuel needs would require the labor of the world's impoverished sugar cane workers.
"Tomorrow the 1st of May is a good day to carry these reflections to the workers and all of the poor people of the world," Fidel Castro wrote in the statement dated Monday evening.
The Cuban economy is healthy, boosted by recent Chinese and Venezuelan investment, reports Logan. But the same entrenched problems that have hobbled this nation for years remain: access to housing, food and transportation.
Those burdens are dealt with daily by Cubans like Alberto Gonzales and his family. "Life is hard," says Gonzales. "You go to sleep every single night and you have to think, what about tomorrow?"
Elsewhere on May Day, riot police charged into crowds of protesters in Turkey, spraying tear gas and kicking and clubbing fleeing demonstrators. Hundreds were arrested.
In the German town of Dortmund, more than 300 leftist rioters set fire to train tracks and vandalized streetcars and buses after a May Day demonstration against right-wing extremism got out of control. More than 130 protesters were detained.
Police used water cannons in Chile when protests turned slightly violent.
Latin American immigrants in the United States also marched against U.S. immigration policies, while other rallies were held in Russia, Japan, Venezuela, Brazil, Pakistan and Bolivia.