Image copyright AFP/Getty Pope Francis has arrived on the Greek island of Lesbos to show support for refugees trying to reach Europe.
The Pope is being met by Orthodox Patriarch Bartholomew. They will visit a camp for more than 3,000 people who are awaiting either processing of asylum claims or deportation to Turkey.
Lesbos has been a key entry point into Europe for migrants in the past year.
Thousands are now stuck on the island after last month's deal between Europe and Turkey to try to ease the flow.
Pope Francis and Patriarch Bartholomew of Constantinople - the spiritual head of the world's Orthodox Christians - will visit the Moria detention centre.
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.
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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.
Pope Francis visited the Italian island of Lampedusa in 2013 to show similar support for migrants after dozens died trying to reach it.
'Terrified'
The Pope was met at Lesbos Mytilene airport by Greek Prime Minister Alexis Tsipras, Patriarch Bartholomew and Archbishop of Athens Ieronimos II.
Image copyright ReutersAt the camp, the Pope will meet unaccompanied migrant children, and will later observe a minute's silence for those who have died making dangerous sea crossings in search of a better life.
On the eve of the 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.
Analysis - BBC diplomatic correspondent Caroline Hawley
Image copyright AFP/GettyThis is a short, but highly symbolic visit designed to draw attention to what the Vatican has described as a difficult situation.
The Pope has repeatedly called for compassion to be shown to people fleeing terror, turmoil and hardship. He will be hoping his trip to Lesbos will stir Europe's conscience.
It comes as the EU's handling of the migrant crisis faces intense criticism from aid agencies and human rights groups. The Pope will meet refugees at a camp where more than 3,000 people are now effectively incarcerated in conditions aid workers say are deplorable.
He will then hold a ceremony to remember those who have died trying to reach Europe's shores. The Pope is coming with a strong moral message - but no solutions.
The Greek authorities have not yet commented on the suicide attempt, which was confirmed by non-governmental organisations on the island.
The man was among four Syrians who have now appealed against their deportation orders.
However, 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.
Refugees and migrants also complain of overcrowded conditions and a lack of food.
Image copyright Getty ImagesKey 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
The Turkey-EU statement in full
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.