Pope Francis is due to visit the Greek island of Lesbos to show support for refugees trying to reach Europe.
He will visit a camp where more than 3,000 people awaiting either asylum or deportation to Turkey are being held
Lesbos has been the main entry point for many among about a million people who entered Europe over the past year.
Thousands are now stuck on the island after last month’s controversial deal between Europe and Turkey to try to stem the flow.
The Vatican insists the pontiff’s five-hour visit is purely humanitarian and religious in nature and should not be seen as a criticism of the deportations.
On the eve of his visit to the Moria detention centre, one Syrian attempted to kill himself 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
This 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.
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
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.