Pope Francis is visiting a detention camp on the Greek island of Lesbos, telling the migrants there that they "are not alone".
The Moria camp holds more than 3,000 people, some of whom may face deportation to Turkey.
Francis said his Greek trip was to witness "the worst humanitarian disaster since the Second World War".
Thousands are now stuck on Lesbos after last month's EU-Turkey deal to try to ease the flow of migrants.
The Vatican insists that the Pope's visit is purely humanitarian and religious in nature and should not be seen as a criticism of the deportations.
Dozens of refugees lined up in the Moria camp to see the Pope, some holding banners asking for help.
Pope Francis met a group of young boys who had made the dangerous overseas journey alone.
As he toured the camp, TV pictures showed one woman kneeling at his feet, delivering an emotional appeal.
A young girl handed him some artwork. The Pope said "Bravo, Bravo", before telling his staff: "Don't fold it. I want it on my desk."
In his speech, the Pope acknowledged "the great sacrifice" the people in the camp had made, saying he wanted to "draw the attention of the world to this grave humanitarian crisis".
Calling on the world to show "common humanity" over the crisis, he told the camp's residents: "Do not lose hope. The greatest gift we can offer to one another is love.