"In one reported instance, one of the activists was made to hang from the ceiling, and according to another testimony, one of the detained women was reportedly subjected to sexual harassment, by interrogators wearing face masks," it added.
According to the testimonies obtained by Amnesty, the activists "were unable to walk or stand properly, had uncontrolled shaking of the hands, and marks on the body."
One of the female activists reportedly attempted to take her own life repeatedly inside the prison.
The report comes in the wake of U.S. President Donald Trump's refusal to use punitive measures against Saudi Arabia over the murder of dissident journalist Jamal Khashoggi in the kingdom's consulate in Istanbul.
In a statement Tuesday, Trump made clear that the benefits of good relations with Saudi Arabia outweigh the possibility that Saudi Crown Prince Mohammed bin Salman ordered the killing.
"Only a few weeks after the ruthless killing of Jamal Khashoggi, these shocking reports of torture, sexual harassment and other forms of ill-treatment, if verified, expose further outrageous human rights violations by the Saudi authorities," Amnesty International's Middle East research director Lynn Maalouf said.
Maalouf urged the international community to take substantive measures to put pressure on Saudi Arabia to immediately and unconditionally release jailed human rights activists.
Riyadh — with the crown prince, in particular — has been under immense international pressure over the Khashoggi case with several states considering imposing sanctions.