At least 1,500 unaccompanied migrant and refugee children stranded in Greece have nowhere safe to stay, with many sleeping rough in the cold and others incarcerated, a charity warned on Saturday ahead of Pope Francis' visit to Lesbos.
Save the Children said Greece must stop locking up children and called on the European Union to help open more safe shelters for them.
"Children ... are sleeping rough in increasingly volatile unofficial accommodation sites, are being incarcerated in detention centers and are slipping through the cracks of the system," said Amy Frost, Save the Children's team leader in Greece.
"They are at risk of violence, abuse and exploitation by people traffickers."
The children - some as young as 10 - come from countries including Syria, Afghanistan and Pakistan. Many have fled war and other violence. Some have traveled alone to Europe, while others have become separated from their families.
Pope Francis will visit the Greek island of Lesbos on Saturday in a trip which aims to draw attention to the frontline of Europe's migrant crisis.
Hundreds of thousands of migrants and refugees entered Europe last year after crossing the Aegean Sea from Turkey to Greece.
But the European Union last month struck a controversial deal with Turkey aimed at closing the Aegean route, and Balkan states have closed their borders.
The closures have left an estimated 2,000 unaccompanied children trapped in Greece, but Save the Children said there were just 477 shelter spaces for them in the country.
The shelters have been full for weeks meaning some new arrivals are put in detention centers and police cells.
Frost said some children were being held for weeks in police custody in "extremely bad conditions".
She criticized the European Union for rushing to close borders and implement the deal with Turkey.
"Pope Francis is telling the world there can be a more compassionate way to deal with the refugee crisis and Save the Children agrees," Frost said in a statement.
The charity said children at the overcrowded centre in Moria - which the Pope is scheduled to visit - were getting sick.
It also had "serious concerns" about the mental and physical wellbeing of nearly 60 children detained in a section of Moria managed by the police.
"The conditions are very dirty, there are not enough beds and they do not have access to legal services," Save the Children spokeswoman Sacha Myers said by phone from Lesbos.
Other children are sleeping rough in Athens - some on park benches - and at Greece's northern border, where Myers said they were at risk of exploitation and abuse.
One 15-year-old Syrian boy, who spent several days sleeping in the rain and mud, described conditions at the border as "wretched".
"I want to cry, but I don't want people to see me crying," he was quoted as saying by the charity. He said he had fled Syria after being jailed and beaten. His boat from Turkey had nearly capsized twice.
The International Organization for Migration said on Friday that 153,362 migrants and refugees had entered Greece by sea in 2016, but numbers had fallen sharply in the last week.SHOW MORE