Crowds, including mothers carrying young children, began streaming towards the tree-lined boulevard in grey light before dawn for the event on Saturday afternoon, as police closed nearby streets where Leo will be driven in an open-top popemobile.
Leo is on a whirlwind four-day, three-city visit to France, where he is seeking to repair the church's standing there after abuse scandals sparked nationwide shock.
The Pope, who has forcefully decried the direction of global leadership in recent months, warned in a speech on Friday at the Paris headquarters of the UN culture and education body, UNESCO, that the UN and other multilateral groups were in crisis.
He also repeated his frequent appeal for the world to reconsider the development of artificial intelligence, telling French leaders that AI could create a future where "a paradise of machines" takes control of the Earth.
Also on Friday, Leo celebrated vespers at the newly reopened Notre-Dame cathedral and then presided over a massive youth vigil at the Paris stadium.
France, a secular nation where the separation of church and state is enshrined in a 1905 law, has witnessed a steady decline in affiliation to the Catholic Church during the post-war era, mirroring a trend in other traditionally Catholic countries.
Although the French Bishops' Conference points to an increase in baptisms of adults, including a record 21,000 at Easter, a report published by the official statistics agency, INSEE, showed that only 25 per cent of French 18-to-49-year-olds identified as Catholic, compared with 43 per cent a decade earlier.
In a visit on Saturday with newly baptised Catholics at the Catholic University of Paris, Leo urged the French church not to be "naively optimistic" about the recent increase in adherents.
"We must recognise, welcome and protect the small flame that the Holy Spirit has kindled in the darkness of the night, the tiny seed that strives to grow even amid the thorns," he said.
Leo heads later on Saturday to Lourdes, home to a famous Catholic shrine.
He will meet seven French victims of abuse by clergy there on Sunday.
