Fidel Castro as hundreds of thousands of Cubans marched through Havana's Revolution Plaza to celebrate May Day, casting new doubts on his recovery and whether he will return to power.

Tuesday marked only the third time in nearly five decades that Castro has missed the sweeping International Workers' Day festivities ¡ª a major celebration here and around the world.

People march during a May Day parade on Revolution Square in Havana May 1, 2007.[Reuters]

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, 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."

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 recent decision to free on bond anti-communist militant Luis 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 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.

On the eve of the march, Fidel Castro mentioned Posada and the five agents in the latest of several editorials he has issued in recent weeks, but focused mostly on his opposition to U.S. plans to use food crops to produce ethanol for American cars.

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 city 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.

Clashes also broke out between police and demonstrators in Berlin, with several reported injuries. Police said they detained rioters, but could not say how many.

Hundreds of thousands of Russians took to the streets as an array of political forces held marches and rallies to mark the holiday. The largest gathering ¡ª estimated by police at 20,000 ¡ª was led by the pro-government United Russia party and a government-approved trade union organization. But thousands also turned out for a Communist Party demonstration.

In the Chinese territory of Macau, police fired shots into the air and used pepper spray to disperse about 1,000 protesters who had veered off a march route and tried to break through police lines. The demonstrators were demanding a crackdown on illegal workers and corruption.

In the U.S., Latin American immigrants marched against U.S. immigration policies, while other rallies were held across South America and Asia.