HAVANA (AP) --- Cubans awaiting a speech by Fidel Castro in the eastern city of Santiago were surprised recently when a substitute appeared on stage --- the president's younger brother, Raul.

Even the announcer on state television was caught off guard when the younger Castro strode to the podium last week and addressed the tens of thousands gathered.

Not that Raul's speech differed markedly from what his brother would have said about the 145th anniversary of the birth of independence hero Jose Marti. Raul dutifully reminded Cubans they're "a fighting people.''

Lately, however, the 71-year-old president has been giving Raul, five years his junior, a more public role in running Cuba's government.

"It's reasonable to speculate that Fidel at this point is now beginning to think about stepping down,'' said Philip Brenner, a Cuban scholar at American University in Washington.

"It could be he's feeling his mortality,'' Brenner said. "It could be that he's tired after 40 years.''

In recent months, Raul Castro has made high-profile visits to China and Italy, as well as more public appearances throughout Cuba.

Dressed in a business suit, he greeted Pope John Paul II at the airport when the pontiff arrived in Santiago and sat in the front row when John Paul celebrated Mass in the eastern city.

As the government's No. 2 man and head of Cuba's defense forces, Raul Castro always has been considered his older brother's successor.

According to University of New Mexico professor Nelson Valdes, Fidel Castro mentioned his brother as his heir apparent at least as far back as 1959, the first year of the revolution, when popular revolutionary hero Camilo Cienfuegos was killed in a plane crash.

Valdes noted that under rules established by the Communist Party in 1965, the party's second secretary --- Raul Castro--- is the legal successor of the first secretary --- Fidel Castro.

Equally important, Raul Castro was at his brother's side when a group of students launched the revolution with a disastrous 1953 attack on Santiago's Moncada Barracks. He also fought in the Sierra Maestra mountains during the revolution that ousted dictator Fulgencio Batista on Jan. 1, 1959.

Fidel Castro underscored the plan for his brother's succession back in October, during the Fifth Communist Party Congress.

Apparently alluding to concerns about his health and Cuba's future leadership, the president said that the party can "count on a relief for all, especially for me'' with the re-election of Raul as second secretary.

"It gives us much security and it gives us great tranquility,'' he added.

The following month, Raul Castro was in Beijing for an official visit to highlight cooperation between the two countries.

Then in December, he stopped in Rome and made a private visit to the Vatican --- just seven weeks before John Paul's historic trip to the Caribbean island.

Despite Raul's legal role as successor, many Cuba watchers over the years have suggested that Raul, aging himself, might be only a short-term replacement for his brother.

But the transition to a younger generation of political leaders already is under way in Cuba, Valdes said, noting that men in their 30s and 40s occupy most top government and Communist Party posts.

Although none of Fidel Castro's possible successors --- including Raul --- has the president's charisma, few, if any, combine public popularity with strong support in the crucial military and party structures.

Raul Castro's rising profile parallels the growing political power of the military. Though it has shrunk in size from its peak of 180,000 in the late 1980s, it has assumed more control over the economy in recent years.

"From time to time Raul has taken prominence'' in Cuba's political affairs, Brenner said. "What's much more telling is that the military is now clearly running affairs in the country. When we think about the future of Cuba, it really is the military.''

The military began establishing farms, businesses and construction brigades after the collapse of communism in Eastern Europe cut off crucial military aid and training in the early 1990s.

In those years, the military was still reeling from a 1989 drug-trafficking scandal that led to the execution of four high-ranking military officers, including Maj. Gen. Arnaldo Ochoa, a hero from Cuba's Angola campaign. Several top military leaders also defected.

Then in 1992, Fidel Castro announced that Cuba no longer would provide military help to insurgent groups or governments abroad, cutting sharply into the military's portfolio.

Since then, the military has regained its prestige and popular support. And through it all, loyalty to Raul has remained high, so high that the armed forces have been dubbed "Raul's Party.''