"This systematic practice endangers the life of people, including young children, that often end up abandoned in the middle of the sea," refugee rights groups say in a letter to Pope Francis ahead of his visit to Greece.
Dozens of refugee rights groups in Greece have urged Pope Francis to help halt illegal pushbacks of migrants carried out by Greek border forces.
"Reports of serious violations of human rights taking place across European borders, to the extent of pushing people back to Turkey, have been corroborated by international organisations," the 36 groups said in a letter to the pope, unveiled at a press conference on Thursday.
"This illegal practice must end, and we ask you to deploy all your influence, in order for this to stop and in order to establish an independent border monitoring mechanism that will investigate these events," the groups including medical charity Medecins du Monde and Jesuit Refugee Service Greece said.
The pontiff will be visiting Greece on Saturday.
NGOs in Greece have repeatedly decried slow asylum procedures and the alleged mistreatment of migrants and refugees in camps and at the EU member's borders, which Greece's government steadfastly denies.
On Wednesday, the New York Times reported than an Afghan interpreter for the EU border agency Frontex had been among 100 people allegedly assaulted and expelled to Turkey in September.
The Greek ombudsman's office is investigating the claim and European Union officials are already separately looking into allegations of maltreatment of the man, who says the guards mistook him for an asylum seeker.
Pope denounces 'hostility'
In November, German MEP Cornelia Ernst said she had found five Somali migrants hiding from a "potential pushback" on the island of Samos.
Migration Minister Notis Mitarachi at the time said he had asked the national transparency authority, an independent watchdog that usually monitors civil service corruption, to investigate.
Pope Francis has denounced the "hostility" and "exploitation" migrants in Europe have faced.
On Sunday, the pontiff will visit the island of Lesvos, one of the main gateways for refugees to Europe.
Lesvos was home to Europe's largest refugee camp until it burned down last year. The pope is scheduled to visit a new camp and speak to some of the residents.
According to UN estimates, around 96,000 refugees and asylum seekers live in Greece.