Pope Benedict XVI has spoken out against abortion at the start of his five-day visit to Brazil, the world's most populous Roman Catholic nation.
This is the pontiff's first visit to Latin America as Pope
The visit is his first to Latin America since becoming Pope in April 2005.
He is to celebrate a series of open air Masses before travelling to Aparecida for the main event of the visit, a conference of Latin American bishops.
There he is expected to touch on the growing challenge the Catholic Church faces from evangelical groups.
10 May: Meets Brazilian President Lula in Sao Paulo, then Church leaders and young people
11 May: Pope leads prayers around Sao Paulo
13 May: Prayers and address to open Latin American bishops' conference at Aparecida
Moments after arriving in Sao Paulo, the Pope stressed the need to respect life "from the moment of conception until natural death".
He expressed his confidence that the bishops' conference would take a strong stand against abortion, which Brazilian President Luiz Inacio Lula da Silva recently described as a public health issue, not a moral one.
Speaking on his flight to Brazil, the Pope was critical of a recent move to legalise abortion in Mexico City.
He also said his main concern in the region was the dwindling Church membership.
According to a recent study, some 64% of Brazilians are Catholic, but this number represents a 10 percentage point fall compared to 10 years ago and contrasts with an upsurge in converts to evangelical churches.
The Pope said the issue of Catholics choosing to join evangelical churches was "our biggest worry". "We need to find a convincing response," he said.
'Peace to all of you'
The Brazilian president greeted the Pope as his plane touched down at Sao Paulo's international airport.
"I extend my greetings to all the peoples of Latin America and the Caribbean in the words of the Apostle: 'Peace to all of you who are in Christ'," the Pope said, speaking in Portuguese.
Crowds waited in the rain to catch a glimpse of the pontiff, who later went to a monastery where he will stay during the visit.
On Thursday he will address a youth gathering in the city's Pacaembu stadium and on Friday canonize Brother Antonio Galvao, Brazil's first native-born saint, during a public Mass.
Then on Sunday Pope Benedict is to open the bishops' conference, the first such meeting for 15 years.
The two-week forum will bring together almost 200 bishops and cardinals from across Latin American and the Caribbean to set out the Church's agenda and policies in the region for the coming years.
Brazil has more Catholics than any other country in the world
The conference comes only weeks after Mexico City's decision on abortion.
Talking to journalists on the plane, Pope Benedict appeared to back Mexico City Church officials who said that politicians who supported the law and medical workers who performed abortions would be excommunicated.
A Vatican spokesman later clarified the issue, saying the Pope did not intend to excommunicate anyone, but those in favour "exclude themselves from Communion".
The subject is also up for debate in Brazil. The health minister has recently said he would like to see discussion on abortion - currently permitted only in limited circumstances - a suggestion that has already prompted a vigorous response from senior clergy.
Our Vatican correspondent, David Willey, who is travelling with the Pope, says that the Church is reluctant for a public discussion on the issue in Brazil.