The gathering at the shrine in the foothills of the Pyrenees is the first major Catholic event in France since priest Jacques Hamel was killed in his church on July 26.
Undeterred by the threat, an estimated 22,000 pilgrims from across Europe, the Middle East and Asia descended on the Lourdes shrine for one of the biggest dates in the Christian calendar, marking the ascent into Heaven of the Virgin Mary.
Security was tight ahead for the annual four-day pilgrimage, which culminated with an open-air mass Monday at the spot where Mary is said to have appeared to a shepherd girl in 1858.
Soldiers in fatigues with automatic rifles slung over their shoulders watched over the ceremony as a helicopter flew overhead.
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In total, 500 security force members were deployed to protect the worshippers, who had their bags searched on entry to the site.
"I think it's important that the soldiers are here just in case anything happened, but at the same time it's really important to pray for peace because so much has been going on," said 21-year-old Leila Bousbaa from Britain.
Cardinal Philippe Barbarin dedicated his homily to his country, which had been "rocked by so much suffering since January 2015 (when a satirical magazine and a Jewish grocery were attacked) and which was again cruelly hit last month."
July's assaults in Nice and Hamel's church in the northern town of Saint-Etienne-du-Rouvray shattered eight months of relative peace in France after the bloodshed at a Paris concert hall and other nightspots in November.
Local church leaders sought to rally the devout Monday, asking them to "pray for France" and light a candle for Hamel.
President Francois Hollande will meet Pope Francis on Wednesday at the Vatican, with presidential sources saying the two plan to discuss the fallout from the priest's death.
The cleric's killers -- French teens Adel Kermiche and Abdel Malik Petitjean -- claimed allegiance to the Islamic State group.