Amnesty International on Tuesday accused Greek authorities of the "torture, ill-treatment and illegal pushbacks" of refugees and migrants to Turkey, arguing that the practice had become "de facto" border policy.
In a new report, the rights group described 21 incidents it said had involved around 1,000 people.
Many had been subjected to violence before being transferred back to Turkey, Adriana Tidona, migration researcher for Europe at Amnesty, said in the report.
"It is clear that multiple arms of the Greek authorities are closely coordinating to brutally apprehend and detain people who are seeking safety in Greece,..." she added.
The pushbacks, on land and at sea, had sometimes involved people apprehended as far as 700 kilometres (435 miles) inside Greek territory, said Amnesty.
"Our research shows that violent pushbacks have become the de facto Greek border control policy in the Evros region," said Tidona, referring to the river bordering Turkey.
"The level of organisation needed to execute these returns, which affected up to 1,000 people in the incidents we documented ... shows just how far Greece is going to illegally return people and cover it up," she added.
Greece has consistently denied it practises illegal pushbacks on either land or sea. It has routinely dismissed the testimonies as "fake news" spread by Turkey.
- Beatings, strip searches -
Earlier this month, in an interview with France24 television, Prime Minister Kyriakos Mitsotakis once again denied asylum seekers were being forced back.
"But...I want to make it very clear, it is our job to defend our borders," he added.
Greece would use "all tools in (its) arsenal with full respect of to international law and human rights" to achieve this, he said.
Amnesty said it had documented incidents from between June and December 2020, several months after the crisis at the beginning of the year when Turkey created a migrant surge by opening its borders.
Most of the people they spoke to said they had experienced or witnessed violence from both uniformed Greek officials and men in civilian clothing.
They described beatings with sticks or truncheons, kicks, punches, slaps, and pushes, sometimes resulting in severe injuries.
Men were often subjected to humiliating and aggressive strip searches, sometimes in sight of women and children, the report said.
Some incidents also amounted to torture, due to their severity and humiliating or punitive intent, said the rights group.
Among those deported to Turkey were a recognised refugee and a registered asylum seeker who had been living in mainland Greece for almost a year, Amnesty added.
One of them, a 31-year-old Syrian man, said police had shredded his asylum card before sending him to the border.
Amnesty called on EU border force Frontex to suspend or withdraw its Greek operations, arguing that the incidents had happened in areas where it has significant numbers of staff.
"The agency cannot therefore claim to be ignorant of the abuses that we, and many others, have documented," Tidona said.