The report claims that thousands of mass hangings took place at the jail between 2011 and 2015.
Amnesty found that up to 50 people were hanged during the executions, which allegedly occurred once or sometimes twice a week.
Most of those hanged were civilians believed to have been opposed the government, Amnesty stated.
Commenting on the alleged executions, Amnesty International research deputy director Lynn Maalouf said: “The horrors depicted in this report reveal a hidden, monstrous campaign, authorised at the highest levels of the Syrian Government, aimed at crushing any form of dissent within the Syrian population.
“The cold-blooded killing of thousands of defenceless prisoners, along with the carefully crafted and systematic programmes of psychological and physical torture that are in place inside Saydnaya Prison cannot be allowed to continue.”
During the investigation, the human rights organisation surveyed 84 witnesses, including former detainees, guards and officials, over the course of a year.
The prisoners’ bodies are transported by the truckload to be secretly buried in mass graves, the statement said.
Last August, Amnesty reported that more than 17,000 people have died in prisons across Syria as a result of inhuman conditions and torture since 2011. However, the figure does not include those hanged at Saydnaya.
Image: Thousands of mass hangings have taken place at Saydnaya during 2011-2015. Photo: © Amnesty International UK.