Pope Francis has criticised plans to legalise drugs in Latin America, warning that the move would not reduce the spread and influence of drug addiction.

The Argentine-born pontiff, who has been on a visit to Brazil since Monday, said education was the way to end drug use during his visit to a drug rehabilitation hospital in Rio de Janeiro run by Franciscan monks on Wednesday.

"The scourge of drug trafficking, which favours violence and sows the seeds of suffering and death, requires of society as a whole an act of courage," Pope Francis told thousands of worshippers on the third day of his week-long visit for World Youth Day, a biennial Catholic Church gathering being celebrated in Rio de Janeiro.

"A reduction in the spread and influence of drug addiction will not be achieved by a liberalisation of drug use, as is currently being proposed in various parts of Latin America."

Some prominent Latin American leaders are moving towards support for the decriminalisation of drugs after decades of violence generated by the unsuccessful war on drug trafficking cartels in the region, the leading source of cocaine in the world.

The 76-year-old pope received another large welcome earlier in the day at the country's main religious shrine where he warned the youth of his continent to avoid the snares of modern life.

Francis landed by helicopter in Aparecida, a shrine of the Virgin Mary that is venerated as the patroness of Brazil, home to the biggest Roman Catholic population in the world.

Calling on youth

"It is true that nowadays, to some extent, everyone, including our young people, feels attracted by the many idols which take the place of God and appear to offer hope: money, success, power, pleasure," he said in Portuguese in his indoor sermon.

"Often a growing sense of loneliness and emptiness in the hearts of many people leads them to seek satisfaction in these ephemeral idols."

The shrine located 260km west of Rio de Janeiro is also the site where Francis, then a cardinal in Argentina, cemented his place as a leader of the Church during a 2007 conference attended by Pope Benedict XVI.

In Aparecida, where tens of thousands gathered for the pope's first public mass of the visit, Francis rode in a white mobile with open sides and a transparent top. Security squads kept the vehicle safely within barriers behind which tens of thousands of ecstatic faithful cheered, sang and waved flags.

At the end of the sermon, as worshippers chanted, "Francisco, Francisco, Francisco," he walked around the basilica and comforted sick people in wheelchairs.

He later joked with the crowd outside, asking their permission to speak Spanish instead of "Brazilian" and led the crowd, as he held the statue of the Virgin, in a prayer.

On Thursday, the pope will visit a slum and then address hundreds of thousands of young Catholics on Copacabana beach as part of the international celebration of World Youth Day.

More than one million young Catholics are expected to descend on Rio De Janeiro on Thursday.