By Joshua McElwee and Leila Miller
BUENOS AIRES, Nov 9 (Reuters) - Vast crowds thronged one of the main boulevards in Buenos Aires on Monday as Pope Leo arrived for an outdoor Mass that city officials said could draw more than 1.5 million people in a first papal trip to Argentina for nearly four decades.
Argentines stretched for miles down the Avenida del Libertador, waving white-and-yellow flags and pressing against security barriers for a glimpse of the pontiff at one of the largest gatherings of his
18-month papacy.
Chants of "Long live Pope Leo!" echoed down the tree-lined street as the pope was driven by in an open-top popemobile under sunny skies.
People had gathered in the early hours of Monday ahead of the Mass. Some normally bustling parts of the city stood near-empty as police cordoned off a security zone.
Vendors along the pilgrims' route were selling souvenirs of the visit, including flags featuring the pope's face.
"I love that (Leo) is visiting us," said Claudia Pardo, 37, waiting with her husband and two young children. "It's a unique, historic moment, and we should listen to his message."
Alejandra Frias, 56, was sleepless after camping out all night for a front-row view to see the pope. "This is an emotional and spiritual moment," she said.
POPE TELLS MILEI TO KNOW LIMITS OF POWER
Leo, 71, is on an ambitious 50-event, three-country tour less than a month after undergoing lung cancer surgery. He visited Uruguay before arriving in Argentina on Sunday and will go to Peru on Wednesday before returning to Rome on November 17.
The first US-born pope spent decades as a missionary in Peru and the Vatican has described the tour as a "homecoming".
Large crowds have turned out during one of the most demanding foreign tours by a pope in decades.
A pope has not visited majority-Catholic Argentina since 1987.
Leo has emerged as a forceful voice on the world stage, lamenting what he sees as a failure of global leadership. On his tour he has urged South American nations to set an example of regional cooperation and patient diplomacy instead of conflict.
On Sunday, the pope addressed Argentine President Javier Milei, a libertarian seen as a close ally of US President Donald Trump. He urged Milei, who has pushed an austerity agenda that the Church has strongly criticized for cutting social services, to not target Argentina's poorest.
He also told Milei, a theatrical figure known for bombastic statements, that leaders should be careful with their words and know the limits of their power.
ECONOMY NOT ONLY MEASURE OF SUCCESS, POPE SAYS
Ahead of Monday's Mass, Leo visited a care center for people with disabilities on the southern outskirts of Buenos Aires, where thousands had also gathered in hope of seeing him.
Jorge Silanes, religious director of the Pequeño Cottolengo Don Orione center that welcomed Leo, said the visit carried added significance because Milei's spending cuts have reduced disability benefits while poverty remains widespread.
Leo said in remarks that centers like theirs "have much to say to our societies, which are often driven solely by the pursuit of economic success."
"A nation's progress is also measured by how it welcomes all people with disabilities and particular needs," said the pope.
The largest event yet in Leo's papacy came in June, when some 1.2 million turned out in Madrid during the pontiff's visit to Spain.
(Reporting by Joshua McElwee and Leila Miller, additional reporting by Maximilian Heath, Jorge Otaola and Hernan Nessi; Editing by Ros Russell and Andrew Cawthorne)
