He forced into signing a confession without being allowed to read it or consult with his parents or a lawyer.
He faced a range of charges from ‘harboring’ protesters, participating in demonstrations and chanting slogans, to ‘concealing the offence of incitement’, setting fire to a car, and throwing petrol bombs.
Zaher was sentenced to beheading in Saudi Arabia's secretive Specialized Criminal Court in 2014 and his sentence was upheld this year, according to the human rights organization, Reprieve.
The prosecutor requested that Zaher be crucified after he is beheaded.
Zaher is currently in solitary confinement awaiting execution, which it is feared could happen at any time.
He is thought to be the youngest person at the time of arrest to be given a death sentence in Saudi Arabia.
Zaher's father, Hassan al-Zaher told The Guardian, 'Please help me save my son from the imminent threat of death. He doesn’t deserve to die just because he participated in a protest rally.'
He said Zaher had told him he had not thrown Molotov cocktails or 'anything similar' at the protest.
Reprieve, which is assisting the juveniles, has asked UK Foreign Secretary Philip Hammond to intervene in Abdullah’s case and those of Ali al-Nimr and Dawoud al-Marhoon, who were also juveniles when they were arrested at protests.
Maya Foa, head of the death penalty team at Reprieve, said: 'Abdullah al-Zaher has been through a horrifying ordeal - arrested at the age of 15 for attending a protest, tortured into a "confession", and now awaiting execution in solitary confinement, far from his family.
Ali al-Nimr
'It is utterly disgraceful that the Saudi authorities are now threatening to carry out his beheading imminently, along with the killing of other juveniles like Ali al-Nimr.
'Those governments who are among the closest Saudi allies - notably the UK and the US - must step in without delay and urge the Saudi authorities to change course.'
Two weeks ago, Saudi media reports suggested the government was preparing to execute more than 50 prisoners in a single day, including those arrested during the protests.