HAVANA — Hundreds of thousands of Cubans bade farewell to Fidel Castro on Monday, pledging allegiance to his socialist ideology and paying tribute before images of the leader as a young guerrilla who went on to rule the country for nearly half a century.

Lines stretched for hours outside the Plaza of the Revolution, the massive plaza where Castro delivered fiery speeches to hundreds of thousands of supporters in the years after he seized power.

There and across the country, people signed condolence books and an oath of loyalty to Castro’s sweeping May 1, 2000, proclamation of the Cuban revolution as an unending battle for socialism, nationalism, and an outsize role for the island on the world stage.

Tribute sites were set up in hundreds of places across the country as the government urged Cubans to reaffirm their belief in a socialist, single-party system that in recent years has struggled to maintain the fervor that was widespread at the triumph of the 1959 revolution.

Many mourners came on their own, but thousands of others were sent in groups by the communist government, which still employs about 80 percent of the working people in Cuba despite the growth of the private sector under Castro’s successor, his brother Raul.

One of the first in line at the Plaza of the Revolution was Tania Jimenez, 53, a mathematician who arrived at 4 a.m. carrying a rose.

‘‘Fidel is everything to us, the soul of this country who gave everything, all his life,’’ Jimenez said, in tears.

Sandra Aguilar, a 48-year-old doctor, said her visit to the memorial had two goals: ‘‘We came to say goodbye to our commander; to reaffirm our support of the revolution.’’

After 10 years of leadership by Raul Castro, a relatively camera-shy and low-key successor, Cuba has found itself riveted once again by the words and images of the leader who dominated the lives of generations.

Since his death on Friday night, state-run newspapers, television, and radio have run wall-to-wall tributes to Fidel Castro, broadcasting nonstop footage of his speeches, interviews, and foreign trips, interspersed with adulatory remembrances by prominent Cubans.

Inside the memorial, thousands walked through three rooms with near-identical displays featuring the 1962 Alberto Korda photograph of the young Castro in the Sierra Maestra mountains, bouquets of white flowers, and an array of Castro’s medals against a black backdrop, framed by honor guards of uniformed soldiers and children in school uniforms.

Castro died two years into the process of normalization with the United States, a country that he spent decades fighting and defying.

Three months after the restart of US commercial flights to Cuba after five decades, the first regular flight to the capital, Havana, touched down Monday at 8:25 a.m. The arrival of the American Airlines jetliner set off cheers from passengers.

Thirty-five minutes later, simultaneous 21-gun salutes sounded in the capital and in the eastern city of Santiago, where Castro launched his revolution in 1953.

All schools and government offices were closing for a second day of homage to Castro on Tuesday, which scheduled to end in a massive rally on the Plaza of the Revolution.

‘‘It’s a terrible sadness. Everyone’s feeling it here,’’ said Orlando Alvarez, a 55-year-old jeweler.

On Wednesday, Castro’s ashes will begin a three-day procession east across Cuba, retracing the march of his bearded rebel army from the Sierra Maestra to the capital. The ashes will be interred on Sunday in Santa Ifigenia cemetery in Santiago, Cuba’s second-largest city.

The mourning scene in Havana was played out on a smaller scale at countless places across the country.

At the Jose Antonio Mella school in Ciudad Escolar Libertad, on the western outskirts of Havana, schoolchildren in golden uniforms and local residents quietly lined up one-by-one to sign the oath. By late morning, there were already more than 1,000 signatures.

Across the street in a community center, 70-year-old retiree Jose Menendez Marquez served as an honor guard as Cubans filed past the photo of Castro. He struggled to keep his chin high as tears rolled down his cheeks.

‘‘There is no one else in the world like Fidel,’’ Menendez said. ‘‘He did so many things for the Cuban people, it’s incredible, innumerable. He delivered us from misery, from hunger.’’

Over the weekend, ordinary people had largely stayed at home, off streets hushed by a prohibition on music and celebration during the nine days of official mourning for Castro.