Fears are mounting of the imminent mass execution of the 14 activists convicted of charges linked to participating in protests in 2011.
For their part, human rights groups such as Amnesty International and Human Rights Watch accused the Saudi authorities of coercing confessions which were later retracted in court and failing to grant fair trials to defendants, including juveniles.
Signed by anti-Apartheid leader Archbishop Desmond Tutu, Yemeni activist Tawakkul Karman, Iranian lawyer Shirin Ebadi and former East Timor president and Nobel laureate Jose Ramos-Horta, the letter released late Friday urged King Salman and Crown Prince Mohammed bin Salman, his son, to "extend the hand of mercy" and refrain from ratifying the death sentences.
Saudi Arabia has one of the world's highest rates of execution. This year alone, it has so far executed 75 people.
In July, the Supreme Court upheld the death penalty for the 14 men, all Saudi Arabian nationals. The sentences must be ratified by the king or the crown prince for the executions to go ahead.
The 14 are all linked to protests in Qatif, Saudi Arabia's Shia-populated eastern province.
The east is also the source of most of Saudi Arabia's oil.