Greek police say they have arrested a French cleric suspected of sexually abusing unaccompanied refugee minors he had sheltered in his house in Thessaloniki, Greece's second-biggest city.

The 52-year-old man, who belongs to the Franciscan Church of France, allegedly molested four homeless Pakistani boys, aged 14 to 18.

The teenagers, who had been sleeping rough around Thessaloniki's main railway station, told police officials that they accepted to stay at the man's home in January after he had offered to provide them with food and housing.

"When questioned, the children said they suffered repeated and persistent sexual abuse by the man," a police spokeswoman told Al Jazeera on Wednesday.

"They say that he took advantage of the fact that they were homeless and without food to sexually abuse them."

The alleged abuse came to light after the boys fled the man's house and one of them found refuge in a Thessaloniki centre for vulnerable migrants.

There, he revealed the abuse to the centre's staff, who contacted police.

Following a search on Tuesday at the man's house in the Toumba area of the northern city, police seized six hard drives and one USB stick, as well as dozens of tablets that fall under the Greek law on addictive substances.

Police on Wednesday said that the French cleric, who has been living in Greece for 12 years, has been charged with sexual abuse and possession and use of drugs.

Three of the boys are currently staying in centres run by NGOs, while one is living at the Diabata refugee camp, according to police.

'Entirely unprotected'

More than 62,000 refugees and migrants are currently stranded in Greece owing to a wave of European border closures and a controversial deal between the EU and Turkey in March 2016.

When the borders closed last year, more than 2,500 children, many of whom had not been registered as unaccompanied, were trapped in Greece, according to Lora Pappa, the head of METAdrasi, a charity working with unaccompanied minors in the country.

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"Currently, despite the big efforts that have taken place, more than 1,200 children remain trapped outside facilities in very difficult conditions - on the Greeks islands or in camps," Pappa told Al Jazeera.

"Sad cases, like the one in Thessaloniki, happen when there is no system of checks in place," she said.

For hundreds of vulnerable, unaccompanied minors in Greece, life is full of risk and uncertainty.

Those who officially register with Greek authorities are taken by police. Despite being entitled to protection, they often find themselves facing prolonged arbitrary detention in custody and abusive treatment.

"This is very problematic ... and often forces many of them to lie to authorities about their age to avoid staying in poor and degrading conditions," Eva Cosse, Greece specialist at Human Rights Watch, told Al Jazeera.

For these children, alone in a foreign country without a parent or an adult responsible for their care, sexual abuse is just one of the many dangers they face.

"These are children who are entirely unprotected. They sleep rough, lack access to education and are exposed to sexual abuse, human trafficking and black labour," Cosse said.

"Greece needs to revise its entire system and services to protect unaccompanied migrant and asylum seeking children."