Pope Benedict XVI has urged young Catholics to resist the "snares of evil" and to promote life "from its beginning to its natural end".
Chanting, dancing and shouting rocked the stadium
The Pope was speaking to a rally of about 40,000 young Catholics at a football stadium in Sao Paulo, Brazil.
He used his speech to reinforce traditional Roman Catholic teachings against abortion and pre-marital sex.
Pope Benedict is making his first visit to Brazil, the world's most populous Catholic country, since taking office.
The Pope entered Pacaembu stadium to a popstar's welcome from his young audience, says the BBC's Gary Duffy in Sao Paulo.
He seemed to enjoy the experience as much as they did, says our correspondent.
Catholics from across Latin America crowded the stadium and thousands more listened to his speech from outside.
Before Pope Benedict took to the stage, some of the young people in the crowd chanted "yes to life, no to abortion".
"Be promoters of life, from its beginning to its natural end," the Pope said.
He warned of the traps of drug use, corruption, violence and the temptations of wealth and power.
"Seek to resist forcefully the snares of evil that are found in many contexts," he said.
The Pope is visiting Brazil at a time when the Roman Catholic church is losing adherents to evangelical churches.
"I send you out, therefore, on the great mission of evangelising young men and women who have gone astray in this world like sheep without a shepherd," he said.
The Pope has already sparked controversy on his five-day visit to Brazil by stressing his strong opposition to abortion, the subject of intense debate in the country.
Some Brazilians are currently pushing for the legalisation of terminations.
Shortly after the Pope's arrival in Brazil, he condemned a move to decriminalise the practice in Mexico City
However, at a meeting with President Luiz Inacio Lula da Silva earlier on Thursday, abortion was not mentioned.
Mr Lula has made it clear that he regards abortion as a public health rather than a moral issue.
The World Health Organisation estimates that illegal abortions numbered more than a million in Brazil last year alone.
An open-air Mass at Sao Paulo's Campo de Marte airport on Friday is expected to attract one million people.
The Pope will canonise Brother Antonio Galvao, Brazil's first native-born saint, at the ceremony.
A smaller crowd of at least 350,000 is likely to attend the second Mass on Sunday in Aparecida, where the Pope will also attend a conference of Latin American bishops.
The first such forum for 15 years will bring together almost 200 bishops and cardinals from across Latin America and the Caribbean to set out the Church's agenda and future policies in the region.