Nusra al-Ahmed, the mother of Ali Mohammed al-Nimr, described the intended punishment as savage and "backwards in the extreme," reports British newspaper the Guardian.
Nimr was 17 when he was detained sometime after joining Shia demonstrators in the eastern coastal city of Qatif seeking equal religious rights in the Sunni-majority country.
The official charges levelled against Nimr include attending a protest, using his phone to encourage further support for the demonstrations and possessing a gun – an accusation which his family denies.
Nimr could be put to death at any moment now that the Saudi supreme court rejected his appeal.
"For other people every hour is composed of 60 minutes, but for me every hour is 60 beats of pain," said Ahmed, who also alleged that her son has been tortured.
"When I visited my son for the first time I didn’t recognize him. I didn't know whether this really was my son Ali or not. I could clearly see a wound on his forehead. Another wound in his nose. They disfigured it. Even his body, he was too thin."
"[When] I started talking to him [he told me that] during the interrogation [he was] being kicked, slapped, of course his teeth fell out … For a month he was peeing blood. He said he felt like a mass of pain, his body was no more."
Under Saudi's sharia penal system, the sentence would involve Nimr being beheaded before his decapitated body is hung from a cross in public.
"I feel that one's very being is repelled at such a ruling … It's backwards in the extreme," said Ahmed. "No sane and normal human being would rule against a child of 17 years old using such a sentence. And why? He didn't shed any blood, he didn’t steal any property. Where did they get it [this sentence]? From the dark ages?"
She asked for intervention from President Obama, whom she called “the head of this world.”
"To rescue someone from harm, there is nothing greater than that. I mean my son and I are simple people and we don't carry any significance in this world but despite that, if he [Obama] carried out this act, I feel it would raise his esteem in the eyes of the world. He would be rescuing us from a great tragedy."
Last week, British Prime Minister David Cameron appealed to Saudi king not to carry out the death sentence on Nimr.
The Saudi ambassador to the United Nations, Abdallah al-Mouallimi, on Friday said the matter was one for Saudi Arabia alone.
"We respectfully request the world to respect our systems and our judicial processes, and our laws and regulations, and not to interfere in the internal affairs of a sovereign state. We believe that we are holding ourselves to the highest standards. If that doesn't please someone here or there, that's their problem, not ours."
