Pope Benedict steps onto Cuba's biggest stage on Wednesday with a public mass in Havana's Revolution Square as he wraps up a trip that began with a blast at communism and will end with a visit with Fidel Castro.

Crowds began gathering during the night to hear the leader of the world's 1.2 billion Catholics speak in the sprawling plaza that Castro, 85, used to fill with big crowds and fiery revolutionary rhetoric in hours-long speeches.

Ten-story high images of Castro's revolutionary comrades Ernesto Che Guevara and Camilo Cienfuegos dominate the treeless expanse of pavement from adjacent office buildings.

Hours before the Mass was to begin, the area was filled with people waving Cuban flags and wearing broad hats and holding umbrellas to shield them from the sun.

Priests prepped the crowd with chants and prayers urging them to rely on their faith at difficult times. The priests also read a biography of Benedict, from his birth in a small town in Germany to his election in 2005 as the successor to the much-beloved Pope John Paul, who made a historic, groundbreaking trip to Cuba in 1998.

Benedicto, thank you for your message of love and tolerance. Cuba needs to unite, without hatreds and old resentments, said Elena de la Vega, a 45-year-old teacher who entered the square carrying a placard saying Welcome Benedict.

I came early and I will hear what the pope has to say even though I'm not Catholic, said state telephone company employee Martiza Alvarez, 40, sitting in the front rows near the altar. We know Cuba has to change and we are doing it. But when is the United States going to change the blockade that is suffocating us. I hope Benedict has something to say about that too.

Benedict is expected to keep up calls for change in Cuba that have punctuated his public comments since his arrival on Monday in the eastern city of Santiago.

The 84-year-old pontiff has spoken of Cuba's need for reconciliation and a more open society, with the Church at its side as a buffer against trauma or social upheaval.

In talks on Tuesday with President Raul Castro, the younger brother of Fidel Castro, he urged a bigger role for the Church and asked that the government consider making Good Friday, the day Christians commemorate Christ's death, a national holiday.

Fidel Castro reinstated Christmas as a holiday ahead of the landmark visit of John Paul that helped improve long-strained Church-state relations.

The Vatican also said it made several humanitarian requests, without giving details but possibly having to do with political prisoners or jailed American contractor Alan Gross, who is serving a 15-year sentence for illegally installing Internet networks on the communist-run island.

FIDEL MEETING POPE

Fidel Castro said on Tuesday in one of his columns, or Reflections, published on the Internet that he would meet briefly with the German pope before he returns to Rome.

With pleasure, I will greet His Excellency Pope Benedict XVI as I did with John Paul II, wrote Castro, who is now mostly retired but still occasionally writes columns and meets with visiting leaders.

He said he decided to request a few minutes of his very occupied time when I learned from the mouth of our Foreign Minister Bruno Rodriguez that modest and simple contact would please (the Pope), Castro said.

The meeting of two octogenarian world figures with widely divergent political views could overshadow what has been a more eventful papal trip than many had expected.

At a time when Church-state relations are the warmest they've been since the 1959 revolution, Benedict has not been afraid to poke the Cuban government in some sensitive places.

On the flight to Mexico beginning his trip on Friday, the pope said communism does not correspond with reality and that Cuba needs a new economic model.

Upon his arrival, he made thinly-veiled references to Cuban dissidents, political prisoners, Cuban exiles and asked the island's patron saint, the Virgin of Charity of El Cobre, to guide the future of this beloved nation in the ways of justice. peace, freedom and reconciliation.

He also praised improved Church-state relations, but said many areas remain in which greater progress can and ought to be made, especially as regards the indispensable public contribution that religion is called to make in the life of society.

His Monday evening mass in Santiago began with a man in the crowd shouting down with communism and being hustled off by security agents.

A BETTER SOCIETY

The pope told the large crowd to strive to build a renewed and open society, a better society, one more worthy of humanity.

President Castro has launched economic changes aimed at strengthening communism for the future, but Marino Murillo, a vice president in the Council of Ministers and the country's economic reforms czar, made it clear that change to Cuba's one-party political system is not in the works.

In Cuba there won't be political reform, he said at a news conference at Havana's Hotel Nacional, the international press centre for the pope visit. We are talking about the update of the Cuban economic model to make our socialism sustainable.

Murillo said the government welcomed all ideas, but would not allow them to be imposed on the country.

In response, Vatican spokesman Father Federico Lombardi said the Church is not trying to impose solutions. We know it is a long road and that the history of Cuba is complex.

The Church would like to get back some of the social and political ground it lost after the revolution, which is why the Pope wants the government to let it do more social work and education.

He also wants to see a Cuban revival of religious faith in general and specifically for the Church, which despite years of diminution remains the island's largest and most socially influential institution outside of the state.

(Editing by David Adams and Philip Barbara)