The American pope waved to the flag-waving and cheering crowd as the popemobile rolled along the parade route towards Place de la Concorde, where the Mass would be held.
The Vatican estimates that around 800,000 people took to the streets to follow the service.
In his sermon, Leo urged those present not to keep the faith to themselves.
Let it overflow, so that those you meet will gain new strength and have their thirst quenched, he said.
The four-day visit is the first papal visit to France in 18 years.
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