The gathering at the sanctuary in the foothills of the Pyrenees is the first major Catholic event in France since priest Jacques Hamel was killed by two jihadists, who stormed his church during Mass on July 26th and slit his throat.
The Feast of the Assumption marks the ascent into Heaven of the Virgin Mary, mother of Jesus Christ. Groups from across Europe, the Middle East and Asia have travelled to Lourdes for the occasion - one of the biggest in the Catholic calendar.
"We've come to pray for peace in the world, which seems to be in chaos," Piet Tarappa, an Indonesian businessman who came from Jakarta with 35 other pilgrims and a bishop, told AFP.
Security was extremely tight ahead for this year's pilgrimage, which culminated on Monday with an open-air mass conducted by Cardinal Philippe Barbarin at the spot where Mary is said to have appeared to a shepherd girl in 1858.
The airspace above Lourdes was closed to flights over the weekend and the streets around the sanctuary were sealed off to traffic. Soldiers in fatigues with automatic rifles slung over their shoulders watched over the ceremony and a helicopter flew overhead.
'Pray for France'
A series of Islamist attacks in France over the past year and a half have rocked the country, with a massacre in the Riviera city of Nice on the July 14th Bastille Day shattering months of relative peace after the devastating attacks in Paris in November.
Noting the sombre mood surrounding this year's Assumption celebrations Lourdes Bishop Nicolas Brouwet said: "Deep down, we realize... that our country is the target, not just the Catholic Church."
Only around seven per cent of French Catholics regularly attend church according to a 2010 survey. Local church leaders have sought to rally the faithful in the face of the recent attacks, asking them to "pray for France".
They have also asked Catholics to visit a church and light a candle for Hamel, whose teen killers claimed allegiance to the Islamic State group. The 85-year-old's murder in Normandy came less than two weeks after a Tunisian - said by investigators to have become radicalized by online jihadist videos - slammed a truck into a crowd leaving a fireworks display in Nice at the other end of the country, killing 85 people.
With tourist arrivals in the country plummeting, authorities in Lourdes had worried that the number of visitors to this year's pilgrimage would be sharply down. Several major public events have been cancelled over security concerns.
But fears of mass cancellations in Lourdes failed to materialize. Instead, organizers noted a late surge in bookings by pilgrims apparently seeking solace in faith.
"It's important to show that life goes on," said Matthieu Guignard, one of the pilgrimage coordinators, told AFP. "It's not because a few fanatics try to sow fear that we should abandon our faith, our beliefs, our way of living."
Lourdes attracts around six million people a year, making it one of the biggest sites of Catholic pilgrimage in the world. Many visitors bring sick or disabled relatives, who come to bathe in a spring in the cave where Bernadette said she saw Mary, believing the water to have healing properties.