The footage, shared by animal rights charity PETA Germany, shows how the animals are used a 'taxis' to transport tourists up more than 500 steep steps to the old town of Firá - despite there being an operation cable car nearby.
Describing the journey as 'arduous', PETA says the donkeys and mules make it four or five times a day, adding: "When not being made to carry extremely heavy loads on their backs, they're tethered in the blazing sun without access to water or shade."
As per veterinary recommendations, donkeys should not carry more than 20 percent of their own body weight, approximately 50 kilograms. Footage shows the animals being ridden by tourists far heavier than this - which can lead to long-term health problems, such as hoof and joint issues.
In addition, the video also shows animals wearing ill-fitting and worn-down saddles which cause painful abrasions and wounds on some animals' abdomens, while inadequate headgear left some with fly-covered head wounds. Animals were seen being whipped, and dragged down steps.
"Owners were found to be denying the animals water, shade from the hot Mediterranean sun, and protection from the elements, which is a violation of Greek animal-welfare laws," says PETA. "Even at night, some donkeys are forced to continue working, as they're used to transport heavy bags of rubbish. Once they're too old or weak to do this work, they're often abandoned to die."
Footage from PETA's investigation
Calling for action
The organization is now calling on the Greek Minister of Tourism and the Mayor of Santorini to end the animal suffering - and the 'widespread violations of animal-welfare laws'.
PETA Director, Elisa Allen, said: "Santorini must ban cruel donkey rides and establish a sanctuary to which these poor animals can be retired.
"In the meantime, PETA is calling on tourists to refuse to support this abuse and to use the port's cable car instead."