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from the Andean region through Central America and Mexico and into the United States, killing rivals, bribing police and threatening judges and politicians.
A crackdown on cartel and criminal activity in Mexico has claimed more than 60,000 lives over the past seven years.
But proponents of legalization say that scrapping drug laws would dry up demand for traffickers and reduce violence. The presidents of Uruguay and Guatemala have suggested that drugs be legalized, as has former Mexico president Vincente Fox.
Francis lashed out at the idea.
"A reduction in the spread and influence of drug addition will not be achieved by a liberalization of drug laws," Francis said. "Rather, it is necessary to confront the problems underlying the use of these drugs, by promoting greater justice, educating young people … accompanying those in difficulty and giving them hope for the future."
As archbishop of Buenos Aires, he issued such stern rebukes that drug dealers threatened the life of one of his most prominent priests, who had become famous for working with addicts in the city's shantytowns.
Unlike his predecessor, Pope Benedict XVI, who had little to say on the topic when he visited Mexico in 2012, Francis has been outspoken on crime. In May, he told organized criminals to "stop wrecking people's lives and repent."
Earlier Wednesday, speaking at Brazil's most sacred Catholic site in Aparecida, Pope Francis called for young people to "maintain hope" in the face of tough economic times and cast aside material trappings to build a better world.
Thousands packed into the huge Basilica of the Shrine of Our Lady of Aparecida in an agricultural region. Tens of thousands more braved a cold rain outside to catch a glimpse of the first pope from the Americas.
Before the Mass, Francis stood in silent prayer in front of the 15-inch statue of the Virgin of Aparecida, the "Black Mary," his eyes tearing up as he breathed heavily. He later carried it in his arms. Francis has entrusted his papacy to the Virgin Mary and, like many Catholics in Latin America, places great importance in devotion to Mary.
Francis asked young people to adopt three postures: hopefulness, joyfulness and "an openness to being surprised by God."
"It is true that nowadays, to some extent, everyone, including young people, is attracted to so many idols that take the place of God: money, success, power (and) pleasure," he said in his homily. "A growing sense of loneliness in the hearts of many people leads them to seek satisfaction in these ephemeral idols."
The pope, popular for his simple style and putting a friendly face on a traditional institution, admonished the young people in attendance to adopt a joyful attitude, telling them, "Christians cannot be pessimists!"
As archbishop of Buenos Aires, Pope Francis walked the streets of the shanties, where he would break bread with the residents in their humble homes and baptize their children.
Now in Rio de Janeiro, Francis was to visit a favela, or shantytown, on Thursday, reflecting a papacy that will have a special focus on the poor and a push to have priests provide more pastoral attention to them.
"In Buenos Aires, he was very concerned about the marginalized areas of his diocese and frequently visited them, including the Villas de Miseria (shanties), which are the favelas of Argentina," says Father José María di Paola, an Argentine priest better known as "Padre Pepe" and famous for his work with drug addicts.
"He put (peripheral places) at the center of his work," adds di Paola, one of 22 priests assigned to live and work the shanties by Pope Francis. "It doesn't surprise me that as pope he would show special attention to them."
Francis has spoken of wanting a "poor church" with a focus on the impoverished and disadvantaged – something reflected in his agenda for World Youth Day in Rio taking place this week. On Thursday, he planned to receive five young criminal offenders at the local archbishop's residence.