Pope Francis has met Cuba's former President, Fidel Castro, after celebrating mass in front of tens of thousands of people in Havana.

The two men discussed world affairs and religion, in what the Vatican called an "informal and friendly" encounter.

Before the meeting, Pope Francis gave a homily in which he urged Cubans to serve each other rather than ideology.

It is the first visit by the Pope to the Communist-ruled island, on a trip that will later take him to the US.

Vatican spokesman Father Federico Lombardi described the meeting between Pope Francis and Fidel Castro, which took place at the former Cuban leader's home, as low-key.

They exchanged books: Pope Francis gave Castro three titles, including a book of sermons by Castro's former teacher, while in return the Pope received Fidel and Religion, a collection of interviews with a Brazilian priest.

Earlier in the day, thousands streamed into Havana's Revolution Square to hear the Pope.

Security services were seen arresting at least three people who were shouting and attempting to distribute flyers at the edge of the square as the Mass got under way.

During his homily the Pope spoke of how "Christians are constantly called to set aside their own wishes and desires, their pursuit of power, and to look instead to those who are most vulnerable".

He also warned against ideology, saying: "Service is never ideological, for we do not serve ideas, we serve people".

Cuba's President Raul Castro, who is not a practising Catholic, attended, as did Cristina Fernandez de Kirchner, president of the Pope's native Argentina.

The Pope praised improved co-operation between the Cuban government and the Church on Saturday, but called for the Church in Cuba to have "the freedom and the means" to pursue its mission.

Both his predecessors, Pope John Paul II and Pope Benedict XVI, visited the island during their papacy.

Pope Francis is due to fly to Washington on Tuesday. He has been credited with helping the recent thaw in diplomatic ties between Cuba and the US.

After his arrival on Saturday, he hailed improving ties between the two countries as "an example of reconciliation for the whole world".

But he also urged both Cuba and the US to "persevere on the path" of detente.