Juan Almeida Bosque, 82, comrade-in-arms of Fidel Castro

One of three surviving rebel leaders who still bore the title “Commander of the Revolution,’’ Mr. Almeida was a major figure in the battle to overthrow Cuban dictator Fulgencio Batista, and through the early years following the Jan. 1, 1959, triumph of the Cuban revolution.

Cuba declared a national day of mourning for today and ordered all flags flown at half-staff.

A bricklayer who began working at age 11, Mr. Almeida was the only black commander among the rebel leaders. After the overthrow of Batista, he served in several military posts and was often seen at public events in his uniform alongside Castro until the Cuban leader fell gravely ill in the summer of 2006 and resigned the presidency in February 2008. Although Mr. Almeida also cut back on work in recent years, he was a mainstay at public events beside Castro’s younger brother and successor, President Raúl Castro.

Mr. Almeida joined the fight against Batista’s dictatorship in March 1952 as a young law student at the University of Havana, where he met Fidel Castro, another aspiring lawyer.

Mr. Almeida was at Fidel Castro’s side a year later, on July 26, 1953, when Cuba’s future president led an armed attack on the Moncada, a military barracks in the eastern city of Santiago. It was a disaster and Mr. Almeida and both Castros were sent to prison. But that failure launched the revolutionary battle that triumphed 5 1/2 years later.

Mr. Almeida and other survivors of the offensive were freed in May 1955 under an amnesty granted to the young revolutionaries. He accompanied the Castros and other comrades to Mexico, where they formed a guerrilla army.

They returned to Cuba in December 1956 on the American yacht Granma and launched their battle from the island’s eastern Sierra Maestra.

Mr. Almeida, the Castro brothers, and Argentine-born Ernesto “Che’’ Guevara were among 16 who survived the landing, in which most of the rebels were killed by government troops.

“No one here gives up!’’ Mr. Almeida shouted to Guevara at the time, coining an enduring slogan of the Cuban revolution and ensuring his place in Cuban communist history. As a guerrilla leader, Mr. Almeida later headed his own front of military operations in eastern Cuba.

His military exploits earned him the title “Comandante de la Revolución,’’ reserved for top leaders of rebel troops under Fidel Castro’s command in the 1950s. Now only Ramiro Valdés and Guillermo Garcia hold the distinction.

In recent decades, Mr. Almeida was a highly visible member of Cuba’s ruling elite. With his full head of white hair and mustache, he sat on the Communist Party’s politburo and served as a vice president on the Council of State, the country’s supreme governing body.

Authorities were organizing a ceremonies today around the country, including Isla de la Juventud, an island off Cuba’s mainland where Mr. Almeida once was imprisoned with the Castro brothers.

Details of his personal life were always closely guarded, and it was not clear how many survivors he had.