RIO DE JANEIRO (AFP) - Pope Francis on Saturday stressed the need to win back believers who have defected to other faiths ahead of a mammoth Rio prayer vigil with young pilgrims.

Addressing Catholic clergy, he called for a "church able to dialogue with those disciples who, having left Jerusalem behind, are wandering aimlessly, alone, with their own disappointment - disillusioned by a Christianity now considered barren, fruitless soil."

Reaching out to those who have left the fold was a key goal of his week-long visit to the world's most populous Catholic country, where the Vatican has been alarmed by the growing strength of Evangelical Protestant churches and spreading secularism.

Pope Francis also exhorted the Catholic church to "seek and serve Christ" in the world's slums, and reminded Brazil's elite to confront social turmoil with "constructive dialogue." In a speech to prominent Brazilians, the Argentine-born pope raised the spectre of social violence in a country rocked last month by massive street protests demanding an end to corruption and a better life.

"Constructive dialogue... (is) essential for facing the present moment," Pope Francis told political, religious and civil society leaders assembled in Rio's Municipal Theater.

"Between selfish indifference and violent protest there is always another possible option: that of dialogue," he said.

Since his installation in March, the first Latin American pope has sought to re-energise Catholics, using his Rio trip to urge young believers to spread the Gospel and "make a mess" in their dioceses.

Hundreds of thousands of young Catholics from gathered here for World Youth Day (WYD) appeared fired up by the charismatic pontiff, who has been drawing colossal crowds since he arrived here Monday.

During a nine-kilometre march to the Copacabana beach venue for Saturday night's vigil, many agreed that the church needed a dose of energy, lamenting that too many have lost interest in an institution hurt by paedophilia and financial scandals.

Some suggested social media could help spread the Gospel while others said young Catholics needed to be more active, join missions and open up about their faith.

"Oh yeah! Shake it up, big time! You have to," said Adrian Antonio Flores, a 31-year-old from the US state of Minnesota who works for a website catering to young Catholics.

"We're alive, we're on fire. When people see others on fire, it's contagious," he said before stopping to pray with some 30 compatriots.

Earlier, the pope used a mass to challenge priests to bring the message of the Gospel to the world's slums as he pressed his drive to revive a struggling Catholic faith.

"It is in the favelas... that we must go to seek and serve Christ," he told thousands of bishops, priests and seminarians from around the world gathered for a mass at Rio's St. Sebastian Cathedral.

"We cannot keep ourselves shut up in parishes, in our communities, when so many people are waiting for the Gospel," said Pope Francis who touts himself as a champion of the poor.

Earlier this week, he visited a Rio shantytown, where he urged residents not to lose hope while slamming inequality and corruption.

To Brazil's ruling class, Francis hammered home that dialogue "is the only way for individuals, families and societies to grow, the only way for the life of peoples to progress." After the speech, he hugged several Amazon natives with pierced noses, body paint, feather headdress and straw skirts, even donning a feathered headgear and posing for photographs.

The young pilgrims meanwhile, many of whom had travelled to Rio from around the globe, lined up to receive snack boxes of tuna salad, chips and juice ahead of the vigil.

On Copacabana beach, they will pray and spend the night before attending Sunday's papal mass marking the close of the Catholic youth fest.

The evening vigil and Sunday's mass were to be held in a field in neighbouring Guaratiba, but rain turned the field into a mud pit, prompting authorities to move the events.

Rio Mayor Eduardo Paes said that up to three million people were expected to attend the mass.