Pope Benedict XVI waves to the crowd as he arrives Wednesday in the popemobile to the Sao Bento Monastery in Sao Paulo, Brazil. The pope is on a five day trip to Brazil, the world's most populous Roman-Catholic country, to inaugurate a regional bishops' conference.

Sao Paulo, Brazil ? Pope Benedict XVI began his first papal trip to Latin America stressing church opposition to abortion Wednesday, suggesting that Catholic politicians in Mexico had excommunicated themselves by legalizing abortion in that nation’s capital.

Benedict, who will inaugurate an important regional bishops’ conference during his trip, also spoke strongly against abortion during his first speech in Brazil. Speaking in Portuguese, he said he’s certain that the bishops will reinforce “the promotion of respect for life from the moment of conception until natural death as an integral requirement of human nature.”

Thousands of faithful waited in the cold rain for a glimpse of Benedict, then chanted “Bento, Bento” and waved flags of different South American nations as he blessed them at the monastery where he is staying.

The Vatican says Benedict will confront major challenges during his visit, such as the church’s declining influence in Brazil, the rise of evangelism, and efforts to expand access to abortions in the region.

Catholic officials have been debating for some time whether politicians who approve abortion legislation as well as doctors and nurses who take part in the procedure subject themselves to automatic excommunication under church law.

The pope was asked where he stands on the issue during the flight to Brazil, in his first full-fledged news conference since becoming pontiff in 2005.

“Do you agree with the excommunications given to legislators in Mexico City on the question?” a reporter asked.

“Yes,” Benedict replied. “The excommunication was not something arbitrary. It is part of the (canon law) code. It is based simply on the principle that the killing of an innocent human child is incompatible with going in Communion with the body of Christ. Thus, they (the bishops) didn’t do anything new or anything surprising. Or arbitrary.”

Church officials later said the pope might have inferred from the question that the Mexican bishops had issued a formal declaration of excommunication for the legislators, something Mexican Cardinal Norberto Rivera has said he has no intention of doing.

On the plane from Rome, Benedict said the exodus of Catholics for evangelical Protestant churches in Latin America was “our biggest worry.”

But he said that the spread of Protestantism shows a “thirst for God” in the region and that he intends to lay down a strategy to answer that call when he meets with bishops from throughout Latin America in a once-a-decade meeting in the shrine city of Aparecida near Sao Paulo.

“We have to become more dynamic,” he said. Evangelical Protestant churches have attracted millions of Latin American Catholics in recent years.

The Vatican also has promised that Benedict will deliver a tough message on poverty and crime during his five-day visit to Brazil – the world’s most populous Roman Catholic country.