Addressing worshippers at his weekly Angelus prayer in St Peter's Square, the 79-year-old pontiff, who is himself the son of Italian immigrants in Argentina, related his visit to a migrant processing centre, where around 3,000 people are being held.
"We greeted around 300 of them, one by one," said Francis, who was accompanied on his visit by Ecumenical Patriarch Bartholomew and Archbishop Ieronymos of Athens.
"There were so many children. Some of these children witnessed their parents and friends dying, drowned at sea.
The Roman Catholic leader then went on to tell of one particular case, "that of a young man, who was not even 40."
More From This Section
"I met him yesterday with his two sons. He was Muslim and told me he had married a Christian, (and that) they loved and respected each other."
But the woman fell victim to Islamist radicals, he said.
"She had her throat slit by terrorists because she would not renounce Christ and abandon her faith," said Francis, calling her a "martyr".
As for her grieving spouse, he said sorrowfully, "this man was crying so much".
Declaring "we are all migrants", Francis used his trip to emphasise that the arrivals were not mere numbers, but people with "faces, names and individual stories.