White silk cushions on which couples tie their wedding rings were handed out as gifts -- complete with the pope's signature in Latin, "Franciscus" -- during an unprecedented event entitled "The Joy of Yes Forever".
The Vatican said there were around 20,000 future brides and grooms attending from 25 countries, all of them enrolled on Catholic marriage preparation courses.
On a sunny day in Rome, couples were regaled with love songs and dance numbers from the stage in front of St Peter's Basilica and even a stand-up comic cracking jokes about chat-up lines before the pope's arrival.
"This pope is really close to the way we live, to modern life, the new family, who we are," said Emmanuelle, a 29-year-old from Nantes in France on a Valentine's holiday with her atheist boyfriend Benoit.
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Michael, 24, said he had left the Catholic Church because of the scandal of child sex abuse by priests but his fiancee Rosa, 21, was still a believer and said she thought the new pope was "a great, positive change".
February 14 was the date of his martyrdom in the year 273.
"Today a lot of people are scared of making definitive choices," the Argentine pope said, adding: "Today everything is changing quickly, nothing lasts long".
"Living together is an art, a patient, beautiful and fascinating journey. It does not end when you conquer each other. In fact it is only just beginning," he said.
"Don't build your house on the sand of sentiments that come and go, but on the rock of real love," he added.
The celebration was not open to same-sex couples as Francis is opposed to gay marriage, even though he has called for a more tolerant approach to homosexuals.