Chants of "Leo, Leo" and "Long live the Pope" echoed down the roughly 2km tree-lined boulevard, from the Place de la Concorde to the Arc de Triomphe, as people pressed against security barriers for a glimpse of Leo as he arrived in an open-top popemobile.
The vehicle paused frequently as people waved white and yellow Vatican flags and Leo blessed people with disabilities, children and babies passed forward by volunteers.
Attendees included Brigitte Macron, the French president's wife, far-right leader Marine Le Pen, who is leading in the polls ahead of next year's presidential election, Prime Minister Sebastien Lecornu and Bernard Arnault, the billionaire chairman of French luxury group LVMH.
Cheers rang out in the crowd as Leo thanked God in his sermon for the chance to visit France on a whirlwind four-day, three-city tour, during which he is seeking to repair the Church's standing after abuse scandals caused nationwide shock.
The Pope told Catholics that only their faith would provide true happiness.
"Only God can fill the emptiness within us," said the Pope.
Attendees began assembling before sunrise to hear the pontiff speak at the mid-afternoon Mass.
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.
He has also frequently spoken out for peace, including in remarks with French leaders at the Elysee on Friday, with his comments criticising the Iran war earlier this year drawing a rebuke from US President Donald Trump.
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 since World War Two, 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 last 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 before the Mass, 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 Catholic shrine. He will meet seven French victims of abuse by clergy there on Sunday.
