HAVANA - There was no sign of a convalescing Fidel Castro yesterday 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 would return to power.

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

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 Revolution Plaza raised questions about whether he was strong enough to run the country.

The 80-year-old leader has missed two other major events since announcing his illness July 31 and temporarily ceding power to his 75-year-old brother, Raul Castro, the defense minister. Raul presided at both those events, as well as at yesterday's parade.

"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 yesterday, 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 still could 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?"

At yesterday's parade, Raul Castro, in his typical olive-green uniform and cap, stood stiffly and smiled. He occasionally waved as marchers in red T-shirts and dark slacks streamed past, clutching plastic Cuban flags, portraits of his brother, and banners denouncing U.S. "imperialism."