By Joshua McElwee and Leila Miller
BUENOS AIRES, Nov 9 (Reuters) - Throngs of people were gathering on Monday on one of the main boulevards in Buenos Aires as Pope Leo was due to arrive for an outdoor Mass that city officials said could attract more than 1.5 million people.
Tens of thousands began streaming down the Avenida del Libertador in the early hours of the morning for the event, likely to be one of the largest gatherings of Leo's 12-day tour across South America and of his 18-month papacy.
Normally bustling streets in the Argentine capital stood nearly empty as police cordoned off large sections of the city to create 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, who was waiting amid the crowds with her husband and two young children. "It's a unique, historic moment, and we should listen to his message."
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 travel on to Peru on Wednesday before leaving the region 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.
Ahead of Monday's Mass, Leo was due to visit a care center for people with disabilities on the southern outskirts of Buenos Aires, where thousands had also gathered in the hope of seeing him.
Jorge Silanes, religious director of the Pequeño Cottolengo Don Orione center welcoming Leo, said the visit carried added significance because Milei's spending cuts have reduced disability benefits while poverty in Argentina remains widespread.
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 and Jorge Otaola; Editing by Ros Russell)
