Athens, Greece (BBN)-Pope Francis is visiting a detention camp on the Greek island of Lesbos to show support for refugees who are trying to reach northern Europe.
The Moria camp holds more than 3,000 people, some of whom may face deportation to Turkey, reports BBC.
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.
However, the Vatican official in charge of migrants, Cardinal Antonio Maria Veglio, has said the EU-Turkey plan treats migrants as merchandise and fails to recognise their dignity as human beings.
'A cemetery'
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 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."
On his plane journey to Lesbos, the Pope told reporters: "This is a voyage marked by sadness... We will witness the worst humanitarian disaster since the Second World War.
"We will see so many people who are suffering, who are fleeing and do not know where to go. And we are also going to a cemetery, the sea. So many people never arrived."
The Pope was met at Lesbos Mytilene airport by Greek Prime Minister Alexis Tsipras, Patriarch Bartholomew, the spiritual head of the world's Orthodox Christians, and Archbishop of Athens Ieronimos II.
At the Moria camp, the Pope will also observe a minute's silence for those who have died making dangerous sea crossings in search of a better life.
Greece's ERT state television reported that Pope Francis had offered to take 10 refugees back to Italy with him. Vatican spokesman Rev Federico Lombardi said he could not comment.
In September, the Pope made space in the Vatican apartments for two refugee families, urging Catholics across Europe to play their part to resolve the crisis.
On the eve of the Lesbos visit, one Syrian attempted to kill himself at the camp after being told he would be deported back to Turkey but was prevented by police.
Migrants later demonstrated, demanding better treatment and to stay in Europe. They complain the camp is overcrowded and there is a lack of food.
Some Syrians on Lesbos say they are terrified by the thought of returning to Turkey because of reports that hundreds of Syrians have been forcibly returned from Turkey to Syria.
Turkey has denied sending back any refugees against their will.
Migrant arrivals in Greece have fallen significantly this year, following the closure of borders and the announcement of the EU-Turkey deal.
In the week to 13 April, arrivals in Greece were 76% lower than the previous week, the International Organization for Migration said.
Pope Francis visited the Italian island of Lampedusa in 2013 to show similar support for migrants after dozens died trying to reach it.
Key points from EU-Turkey agreement
Returns: All "irregular migrants" crossing from Turkey into Greece from 20 March will be sent back. Each arrival will be individually assessed by the Greek authorities
One-for-one: For each Syrian returned to Turkey, a Syrian migrant will be resettled in the EU. Priority will be given to those who have not tried to illegally enter the EU and the number is capped at 72,000
Visa restrictions: Turkish nationals should have access to the Schengen passport-free zone by June. This will not apply to non-Schengen countries like Britain
Financial aid: The EU is to speed up the allocation of €3bn ($3.3 bn; £2.3 bn) in aid to Turkey to help migrants
Turkey EU membership: Both sides agreed to "re-energise" Turkey's bid to join the European bloc, with talks due by July
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