With then-President Néstor Kirchner in attendance, the former Archbishop of Buenos Aires delivered a blistering sermon in 2004 lamenting the "exhibitionism" and "strident pronouncements" of leaders. Mr. Kirchner left enraged and never attended the archbishop's services again, calling him the "spiritual leader of the opposition."
Arriving in Brazil on Monday for his first major overseas trip as pope, the 76-year-old Argentine is now on a far bigger stage. While his chief mission is to revitalize the Catholic Church, the world's first Latin American pope is shaping up as a player in Latin American politics as well. His growing popularity in his home region gives him the power to bring prestige—as well as criticism—of Latin American leaders to a far bigger audience.
"He is comfortable using the bully pulpit to call out a wrong when he sees a wrong and to urge politicians to do their jobs, to help the poor, which is a great thing for a 21st-century pope," said Riordan Roett, who directs the Western Hemisphere program at Johns Hopkins University.
To be sure, there are limits to the role Pope Francis may play in Latin America. For starters, the traditionally Catholic region's flock has declined in recent years. The pontiff also must grapple with church scandals such as sex abuse and corruption at the Vatican Bank.
Still, Pope Francis' popular appeal was easy to see as thousands of Catholics, mostly young, mobbed downtown Rio late Monday after he touched down, hoping to grab a glimpse of the recently elected pontiff as he started a weeklong visit to Brazil.
The pope, dressed in simple white gown, made his way through the cheering crowds in a white, open jeep, reflecting his decision to eliminate the popemobile's bulletproof windows.
"He's a very simple, common man," said Carmelia Silva, an teacher at a Jesuit school in Rio. "That's why people want to get close to him."
Massive protests that shook Brazil in June lingered on Monday, raising security concerns during the Pope's visit.
Late Monday, police fired tear gas to clear a small group of antigovernment protesters outside the government palace where the pope met Brazilian leaders. When he left the palace, the Pope deviated from his planned route and took a helicopter to avoid the commotion, a Vatican spokesman said.
Brazil's military also said it found and defused a small homemade bomb near the Catholic shrine of Aparecida where Pope Francis is set to visit this week.
Latin America's leaders are taking care to get on the pope's good side. Argentine President Cristina Kirchner, who succeeded her husband after his 2010 death, converted almost overnight into an ally after years calling him an opponent and battling him over issues including her support for gay marriage.
After he was elected pope in March, Mrs. Kirchner's backers put up posters around Buenos Aires linking the pontiff to the president's party. Mrs. Kirchner visited Pope Francis at the Vatican in Rome and gave him a gourd used for sipping Argentina's traditional mate tea. He kissed her cheek.
"Cristina's turnaround was amazing," said Michael Shifter, president of the Inter-American Dialogue. "Francis doesn't pull any punches, so [Latin American leaders] all have to take care."
Few leaders have more at stake than Pope Francis' host in Brazil, President Dilma Rousseff. The one-time Marxist guerrilla's approval ratings have dropped sharply in recent weeks amid mass protests against everything from government corruption and police brutality to overspending on stadiums to host next year's soccer World Cup. Her approval rating fell to 30% at the end of June from 65% in March.
In a speech beside Pope Francis late Monday, Ms. Rousseff called for a partnership with the church in combating issues such as poverty and hunger. "We have an enemy in common," she said.
Pope Francis' trip presents both an opportunity and a risk for Ms. Rousseff. Association with the pope could help her popularity. But she could be damaged if Pope Francis makes critiques that hit close to home.
"Francis is a highly, highly popular figure in Brazil, and there is an opportunity for [Ms. Rousseff] to tag along, though she must have the political sensitivity to do this in a way so that it doesn't backfire," said Paulo Sotero, the director of the Brazil Institute at the Woodrow Wilson Center in Washington, D.C.
Pope Francis is returning to Latin America at a time when the region lacks a strong unifying voice, which may afford more latitude for his messages of humility and service to the poor. Venezuela's firebrand leftist Hugo Chávez, who had a regional role as the head of an alliance of leftist leaders, died this year. Brazil's former President Luiz Inácio Lula da Silva, who also enjoyed regional popularity, has receded from the political scene, following treatment for cancer and corruption convictions of some aides.
Pope Francis has common ground with many of the region's left-leaning governments. He is a critic of "neo-liberal" economic policies promoted by the International Monetary Fund, and which leaders from Brazil to Ecuador blame for increasing sovereign debt and social inequality.
Still, he has criticized the kind of personality politics often practiced by Latin populists and eschewed the radical leftism that helped form regional leaders such as Ms. Rousseff, who was jailed in her youth for participating in a Marxist rebel group.
The pope already appears to be stepping into the mediator role the church often plays in Latin America. Recently, he moved to heal the ideological schism of Latin America's recent bloody past, by unblocking the sainthood process of Oscar Romero, the El Salvadoran archbishop killed by a right-wing death squad during mass in 1980.
Weeks into his papacy, the pontiff sought to calm tensions in Venezuela after a contested election to replace Mr. Chávez in April started devolving into deadly protests. He issued a statement calling for dialogue.
Mr. Chávez had been a harsh critic of the church and famously stated that Jesus would whip some priests for their unholy behavior if he were here. Nicolás Maduro, who ran in the election as Mr. Chávez's handpicked successor, was also a church critic. But Mr. Maduro responded quickly to Pope Francis' statement with a tweet, as did opposition candidate Henrique Capriles.
Mr. Maduro's tone toward the church seems to have softened. In June, he traveled to the Vatican to meet Pope Francis, saying after the meeting, "It was an extraordinary conversation with a human being full of humility."