At least 1500 unaccompanied migrant and refugee children stranded in Greece have nowhere safe to stay, many sleeping rough in the cold and others incarcerated, a charity has warned ahead of Pope Francis' visit to Lesbos.
Save the Children said Greece had to 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 centres 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 only 10 years old - come from countries including Syria, Afghanistan and Pakistan. Many have fled war and other violence. Some have travelled alone to Europe, while others have become separated from their families.
Pope Francis was to visit the Greek island of Lesbos on Saturday in a trip aiming to draw attention to the frontline of Europe's migrant crisis.
Border closures have left an estimated 2000 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 centres and police cells.
"SERIOUS CONCERNS"
Frost said some children were being held for weeks in police custody in "extremely bad conditions".
She criticised 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.
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 well-being 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.
