RIO DE JANEIRO/SAO PAULO  (Reuters) - Five months after becoming the first non-European pontiff in 13 centuries, Pope Francis is on his debut trip abroad in Latin America for a gathering of young faithful in Brazil, the world's largest Roman Catholic country.

Francis, an Argentine confirmed shortly after he assumed the papacy, he would head to his home region for World Youth Day, a weeklong visit that began on Monday.

The biennial event, expected to draw more than a million visitors to Rio de Janeiro and nearby sites, is part of the Church's effort to energize Catholics at a time when inroads by rival faiths and secularism continue to thin its flock.

It also comes at a time when youth in Brazil, galvanized by a series of mass protests that erupted across the country last month, have grown increasingly vocal about their discontent with the status quo in Latin America's biggest country.

Francis' efforts to put a simpler stamp on the papacy and his origins in the ranks of the region's priests have raised hopes for many of the faithful in Brazil.

So has his stated intent to root out the child sexual abuse scandal that has rattled the Church and to place more emphasis on pastoral work - day-to-day ministry, as opposed to the doctrinaire focus favored by Benedict, his predecessor.

"This new pope is more simple," said Amanda Martins, a 21-year-old Catholic attending mass near São Paulo this month. "Maybe his visit can strengthen the Church."

REINFORCEMENT

The Church could use reinforcement in Latin America, a historical stronghold. Though still home to more Catholics than any other region, the fervor has faded because of growth by protestant faiths and because consumer-focused city-dwellers have eclipsed the rural populace in which Catholicism took root.

The percentage of Latin Americans who identify as Catholic fell to 72 percent in 2010 from about 90 percent in 1910, according to the Pew Forum on Religion and Public Life.

"The Pope is coming as a pastor," said Cardinal Raymundo Damasceno, archbishop of Nossa Senhora de Aparecida, a sanctuary near São Paulo that Francis will visit. "His message will touch on the problems of the people and seek to shed light on the challenges ahead for the Church and society."

For Brazil's 120 million Catholics, challenges abound.

As a near decade-long economic boom gives way to slower growth, the country is roiling from popular protests. More than 1 million people hit the streets in June to protest against everything from rising prices to bad government and poor public schools and hospitals.

The biggest demonstrations subsided, but small protests continue, occasionally turning violent. Late on Wednesday, police clashed with looters and vandals who smashed windows and burned garbage in two upscale Rio neighborhoods.

The initial protests successfully used a major international soccer event as a backdrop, contrasting the billions of dollars needed to host the 2014 World Cup with shortfalls in public services. Authorities hope the papal visit will not provide a similar stage.

Along with expected protests by feminists, gay-rights groups and others who oppose the Church's stance on social issues, authorities fear demonstrators will focus on the 350 million reais ($158 million) that organizers say the event could cost. Most of that will be paid by participants and sponsors, organizers say, but millions more will be needed from public coffers to ensure security, transportation and readiness.

SINS OF OFFICIALS

"Pope Francis isn't to blame for the sins of Brazilian government officials," said Rio Mayor Eduardo Paes this week.

The Vatican itself has played down security concerns.

Francis plans to travel around the city in an open jeep, not the bullet-proof vehicles used since a would-be assassin shot and wounded Pope John Paul II in 1981.

"We think everyone will understand that the pope's message is one of solidarity and peaceful coexistence," Father Federico Lombardi, the Vatican spokesman, said.

Just in case, federal and local officials are deploying 22,000 soldiers, police and other security officials. To help ease traffic in Rio, a city routinely crippled by gridlock, authorities declared holidays for the peak days of the visit.

In addition to a seaside speech in Rio's Copacabana neighborhood and a stroll through a well-known shantytown, Francis is scheduled to meet with President Dilma Rousseff and travel to the Aparecida shrine and a remote pasture west of Rio, where the final two days of the gathering will be held.

Francis has asked to stay in a room in a church residence similar to that of other priests, not a big suite organizers had prepared.

Still, the event is a major spectacle.

The altar being assembled in Copacabana, a terraced stage with four circular mini-altars and a backdrop of giant screens, dwarfs the stage used by the Rolling Stones and others for major concerts on the same spot in recent years. About 10,000 buses will ferry worshippers to and from the pasture in rural Rio.

"It's a spectacle to show the world that the Church still exists and is very much alive," says Leonardo Boff, a prominent Brazilian theologian and former Catholic priest.

Rio's beaches and other tourist hot spots are already buzzing with Catholic visitors, many of them donning colorful T-shirts and flags from their countries of origin.

Even the sand sculptors along the Copacabana shoreline, known for suggestive beach statues of near-naked women, have gotten into the spirit.

"It wasn't cool to have that big butt sticking out," said Ubiratan dos Santos, a sculptor, telling a local newspaper why one of his creations now wears a skirt instead of her usual thong.

(Additional reporting by Philip Pullella in Rome and Rodrigo Viga Gaier in Rio de Janeiro; editing by Todd Benson and Mohammad Zargham)