The head of communist Cuba's powerful labor union called for more efficiency and harder work in the face of rising world fuel and food prices as hundreds of thousands of workers joined the traditional May Day march on Thursday.

The secretary-general of the Cuban Workers Confederation, Salvador Valdes Mesa, also exhorted workers massed in Havana's broad Revolution Plaza to adhere to the principles of ailing ex-president Fidel Castro, the founder of the island's 50-year-old revolution.

"We Cubans have great challenges before us," Valdes told the crowd, saying workers needed to rout out "inefficiencies and weaknesses" in the workplace.

Many wearing red or white T-shirts, the marchers flowed down a major boulevard and past a parade stand, waving Cuban flags and hoisting banners exalting the Castro brothers. While Fidel's image was more common, for the first time there were some posters with photographs of Raul Castro, who was named president on Feb. 24.

Wearing an olive green uniform, Raul Castro smiled and waved from the stand but did not speak as he oversaw the first major event on the communist-run island since he permanently assumed the presidency. Some Cubans had hoped the government would use May Day to announce more reforms.

Since succeeding his brother Fidel, the 76-year-old Raul has lifted restrictions for ordinary Cubans on cell phones, computers and DVD players. They can now also rent cars and stay in luxury hotels on the island _ if they can afford it.

The new administration recently began letting small farmers find better uses for fallow government land, and is making it easier for state workers to own homes they previously rented through their jobs. Increases in small government pensions and court employee salaries were also announced earlier this week.

Many hope the government will soon ease foreign travel restrictions and authorize salary increases for more state employees _ or even strengthen the national currency, now worth 24-to-1 against the U.S. dollar.

The average state salary is just US$19.50 (euro12.50) per month, though health care and education are free, basic food is subsidized and most people do not have to pay for housing.

May Day is celebrated worldwide, but few gatherings rival the event in communist Cuba, where hundreds of thousands march every year.

It was the second consecutive year and only the fourth time in nearly five decades that Fidel Castro has missed May Day festivities since the 1959 revolution.

The 81-year-old, who has not been seen in public since undergoing emergency intestinal surgery in July 2006, published an essay several hours before the celebration denouncing a movement by several states in Bolivia that seek autonomy from President Evo Morales' leftist government.

Castro suggested that Washington plans to use the movement to oust his ally Morales with "some anti-patriotic military sectors."

"Tthe sister Republic of Bolivia ... is days, or even hours, from suffering dramatic events," Castro wrote in the essay published on government Web sites.