Mexican lead researcher Dr Gerardo Ceballos, from the Universidad Autonoma de Mexico, warned that humans could one day follow in the footsteps of the dinosaurs.
"If it is allowed to continue, life would take many millions of years to recover, and our species itself would likely disappear early on," he said.
Mass extinctions have occurred on five occasions throughout the history of life on Earth. The last event happened 65 million years ago when a giant meteor smashed into the planet, altered the climate, and wiped out the dinosaurs.
The scientists used a conservative approach to calculate a natural "background rate" of two mammal extinctions per 10,000 species per century.
This was the rate of diversity loss that should be expected between mass extinctions.
Given the number of species that had vanished over the last 100 years, it would have taken between 800 and 10,000 years for so many creatures to disappear if the background rate had applied.
Writing in the journal Science Advances, the researchers concluded: "Our analysis emphasises that our global society has started to destroy species of other organisms at an accelerating rate, initiating a mass extinction episode unparalleled for 65 million years."
The authors said it was still - just - possible to avert a "dramatic decay of biodiversity" through intensive conservation, but time was running out.
They wrote: "Avoiding a true sixth mass extinction will require rapid, greatly intensified efforts to conserve already threatened species, and to alleviate pressures on their populations - notably habitat loss, over-exploitation for economic gain and climate change.
"All of these are related to human population size and growth, which increases consumption (especially among the rich), and economic inequity."
