What happened next would change his life forever.
Al-Nimr was taken to a local police station, where he was beaten so badly he had to be transferred to a hospital, his lawyer said.
Initially, al-Nimr was hit with relatively minor charges related to his participation in the widespread 2011 to 2012 Arab Spring demonstrations against Shia repression in the eastern part of the country, where most of the population resides.
But when his uncle, the reformist Shia cleric and protest leader Sheikh Nimr al-Nimr, was arrested, prosecutors ramped up their case. Instead of minor infractions related to the protests, al-Nimr now stood accused of joining a terrorist organization, throwing Molotov cocktails and arson.
After being moved to an adult prison at the age of 18, he confessed to a string of crimes under extreme torture, according to his lawyer, Taha al-Hajji. At trial, al-Nimr rescinded his confession, but this was ignored by the presiding judge, according to al-Hajji.
Then, in May 2014, al-Nimr was sentenced to death by "crucifixion," contrary to Article 37 of the U.N. Convention on the Rights of the Child, which states that no individual should be sentenced to death for crimes committed under the age of 18. Saudi Arabia is one of the 196 countries that has ratified the CRC.
Al-Nimr, now 24, is not alone. In fact, he is one of three Saudi Arabian men known to be on death row who were arrested and charged with crimes allegedly committed when they were minors.
The cases of al-Nimr, Abdullah al-Zaher, 23, and Dawood al-Marhoon, 24, follow the brutally familiar pattern of arrest, torture and then, once they could be tried as adults, being sentenced to death for crimes against the state committed before they turned 18, according to Reprieve, the human rights advocates campaigning for their release.
The U.N.'s Working Group on Arbitrary Detention, an independent body that investigates "cases of deprivation of liberty," stated in 2017 that the Arab Spring protests were "recognized by the international community as peaceful" and that the trio "did not engage in any violent or hostile acts." The men were not arrested during the protests -- only after -- and no warrants were presented at the time of their arrests.
The Saudi Ministry of Justice has not responded to ABC News' request for comment for this story.
According to Reprieve case files, al-Zaher and al-Marhoon underwent an ordeal nearly identical to al-Nimr. Reprieve said that on March 3, 2012, 15-year-old al-Zaher was arrested, beaten, shot at and held in solitary confinement in Dammam after allegedly participating in protests.
"In prison, Saudi police tortured Abdullah -- including beating him with wire iron rods -- and forced him to sign a paper that he had not read, without allowing him to speak to his family or a lawyer," Reprieve wrote.
In May of that year, at 16, after refusing to "spy" on protesters, al-Marhoon was arrested in Dammam Central Hospital, where he was receiving treatment for injuries sustained in a traffic accident, Reprieve said.
"The Saudi authorities tortured him for weeks and refused to allow him to communicate with anyone on the outside world," the organization said. "For two weeks, Dawood's family had no idea where Saudi authorities were holding him, and he was prevented from speaking to a lawyer."
Both were transferred to adult prison at the age of 18 and allegedly tortured into confessing to the crime of "herabah," meaning banditry, according to the U.N. and Reprieve. The men were tried jointly and sentenced to death by crucifixion on Oct. 21, 2014, according to Reprieve case files.
Inhumane treatment of three young men in government custody, as reported by an unnamed source, was relayed in a report by the Working Group on Arbitrary Detention in 2017, in which the men are referred to as "Minors A, B and C," but their birth dates match those of al-Nimr, al-Marhoon and al-Zaher.
The Working Group concluded that their detentions were "arbitrary," and that "the adequate remedy would be to release all three minors immediately and to accord them an enforceable right to reparations, in accordance with international law." Their appeals were rejected in 2015.
In its response to the Working Group, the Saudi government denied the allegations of torture, unfair trial and trumped-up charges and said "its criminal justice system provided all the guarantees of fair trial and fair procedures that were consistent with its international obligations in the field of human rights under the general principles of an independent judiciary."
The government also said that the men were "fully fledged adults" and that there were "no violations of its obligations under the Convention of the Rights of the Child."
All of the men were tried and sentenced in the non-Sharia Specialized Criminal Court, or SCC, which was set up in 2008, purportedly to deal with terrorism and state security offences. Generally, the law in Saudi Arabia is Sharia, which is based on Islamic traditions.
According to the American Bar Association, the court has been acting primarily in concert with the "longstanding pattern of misusing counterterrorism resources to stifle dissent" while failing to "effectively investigate and prosecute terrorism financing emanating from the Kingdom."
"Indeed, in several judgments reviewed, Shia protesters were given the death sentence solely on the basis of confessions alleged to have been produced through torture," the Bar Association wrote.
In a review of seven such cases, including al-Nimr's, the Bar Association found that "the SCC convicted every defendant on the basis of their 'confessions' alone, absent any additional evidence of the alleged crimes and although such evidence should have been readily available based upon the prosecution's assertions."
Al-Hajji painted a grim picture of the court as well: armed guards at every turn, no one but the defendant's attorney and a family member allowed in, surrounded by heavily fortified concrete walls and barriers.
"When I first saw Ali, he looked different than he did in the pictures that his mother always posts of him," al-Hajji, who attended the first hearing at the SCC alongside al-Nimr's father, told ABC News. "He was pale, his head was shaved, and his nose looked swollen and unnatural. I learned later this was because of the treatment he was exposed to during detention."
Al-Hajji and Reprieve allege that al-Nimr, as well as al-Zaher and al-Marhoon, were coerced into signing false confessions and denied fair trials. Both al-Hajji and al-Nimr's father attempted to visit Ali in prison "many times," the lawyer said, but were always refused entry, from when he was first assigned to the case in until his client's sentencing in May 2014. Al-Hajji said the prison would claim it had not received an order from the court, while the court would tell him it had in fact sent the order. This wrangling meant al-Nimr's own lawyer was never allowed to meet Ali in person during his case.
Unable to present a defense to the court, al-Nimr was sentenced to death without a right to a fair trial, al-Hajji alleges.
Al-Hajji, along with numerous other human rights activists, left Saudi Arabia to seek asylum in Europe in March 2016. That was two months after al-Nimr's uncle was killed along with 46 other prisoners in January, in a mass execution.
'Their situation is very dangerous'
The official status of al-Nimr, al-Zaher and al-Marhoon is "at risk of imminent execution," according to the U.N., having not been among the 37 people killed in a mass execution in April of this year. Six of the 37 executed then were minors at the time of their arrests, according to the European Saudi Organisation for Human Rights, a Europe-based organization that documents human rights abuses in Saudi Arabia.
The status of "at risk of imminent execution" is used when a defendant has exhausted all legal remedies. Since there is no notice of an execution date in Saudi Arabia, a defendant in that position is always at risk of execution.
Al-Zaher and al-Marhoon are being held in solitary confinement in Riyadh's Al-Ha'ir Prison, while al-Nimr is being held in the General Directorate of Investigations Prison in Dammam, according to Reprieve. The psychological impact is "horrible" for the men and their families, al-Hajji said.
"Ali, Abdullah and Dawood's position is now even more difficult than someone who is scheduled to die, because that person knows he may be executed at any moment," al-Hajji said. "Their situation is very dangerous. Their families live in constant dread, never knowing who has been killed. In the midst of such terrifying and brutal anticipation, some of them may wish for death to end these horrible feelings."
Since 2014, all three men have awaited their executions and "crucifixion," which in Saudi Arabia means the public display of the body after beheading. According to Bloomberg, crucifixion is rare in Saudi Arabia. A man from Myanmar was executed and crucified in 2018 after being accused of stabbing a woman to death, the outlet said.
Saudi Arabia has one of the highest numbers of executions each year worldwide, according to ESOHR, with 149 in 2018 and 122 so far in 2019.
Calls from the United Nations for Saudi Arabia to amend its death penalty were heeded to a certain extent: in 2018, the country amended its juvenile code, formally commuting the death penalty for those who committed crimes under the age of 15 to "placement in a home for a period of no more than 10 years."
But U.N. human rights experts said the change wasn't enough, arguing "children should never be subject to the death penalty." Under the the new rule, the death penalty is still in effect for al-Nimr, al-Zaher, and al-Marhoon, who were 17, 16 and 15, respectively, at the time of their arrests.
Those men, Mujtaba al-Sweikat, Abdulkarim al-Hawaj and Salman Qureish, had been charged with offenses that U.N. human rights experts "previously have considered to represent criminalization of the exercise of fundamental rights, including freedom of assembly and expression, when they were aged less than 18 years old."
The execution of al-Sweikat, a prospective U.S. college student arrested in the kingdom at the age of 17, when he was on his way to attend Western Michigan University, drew especially sharp condemnation from Congresswoman Rashida Tlaib, D-Mich.
"Saudi Arabia's ruler MBS [Mohammed bin Salman] tortures & executes children," she posted on Twitter on April 24. "Already this year, he has killed 100 people. At least 3 today were arrested as teenagers & tortured into false confessions. He killed them for attending protests! Think about that."
The Arab Spring
The current wave of repression dates back to the 2011 Arab Spring and the former King Abdullah, according to Ali Adubisi, director of ESOHR, who himself spent over a year in prison between 2011 and 2012, before he fled.
"In 2011, the eastern region of Saudi Arabia was affected by the Arab Spring, especially as it suffered from economic and social deprivation as well as religious discrimination," he said. "The Saudi government's response to these moves was violent. Live weapons were used against demonstrators and participants, and then large-scale arrests were carried out … in prisons, the situation was also worse, more violent as torture and violence increased."
The policies have been continued under the current power axis of Crown Prince Mohammed bin Salman and his father, King Salman.
Despite the pair's initially promising a reform agenda, called Vision 2030, the humanitarian crisis in Yemen and the murder of Washington Post journalist Jamal Khashoggi cast doubts on that effort.
International pressure is key
Sherif Azer, who leads Reprieve's Middle East team, said that the roles of the U.S. and U.K. are essential if there is going to be a change in the cases.
The Trump administration in particular has drawn criticism for failing to condemn Saudi Arabia's human rights record, with the president's saying bin Salman has done "really a spectacular job" at the G-20 summit and defending the kingdom amid its war in Yemen and the slaying of Khashoggi. The administration also approved two nuclear technology transfers to the kingdom after Khashoggi's killing, which a U.N. report said was the work of Saudi leadership.
"While Mohammed bin Salman glad-hands world leaders at the G-20 summit, young men arrested as teenagers sit on Saudi death row, wondering if they will be beheaded next. The Saudi regime appears to believe it is exempt from international law, and can execute children with impunity," Azer told ABC News. "Its western partners, particularly the U.S. and U.K., must stress that there will be consequences for such lawless and repugnant acts."
In a statement, a U.S. State Department spokesperson condemned the prospective executions.
"We call on the Government of Saudi Arabia, and all governments, to ensure that no death penalty is imposed in any case involving a defendant who was a minor at the time of the arrest or alleged crime," the spokesperson said. "We have spoken out publicly about our concerns, including in the Human Rights and International Religious Freedom reports, and continue to do so in our private diplomatic engagements as well."
In the 2018 report, the State Department mentioned the cases of the three men and said that "senior embassy and consulate general officials continued to press the government to respect religious freedom, eliminate discriminatory enforcement of laws against religious minorities, and promote respect and tolerance for minority religious practices and beliefs."
Secretary of State Mike Pompeo also just unveiled the Commission on Unalienable Rights at the State Department to reinvigorate the country's global approach to human rights in pursuit of a "moral foreign policy." Yet in an article for The Wall Street Journal, Pompeo highlighted the cases of Iran and Cuba but made no mention of Saudi Arabia.
So far this year, 122 people -- six were minors at the time of their sentencing -- have been executed in Saudi Arabia, double the number of executions carried out by this time last year, according to the EOSHR.
In the U.K., the High Court ruled last month that continued arms sales to Saudi Arabia were illegal in the context of the Yemen conflict. A U.K. Foreign Office source told ABC News that the country is opposed to the death penalty in all cases where international standards are not met, and no aspect of its commercial relationship prevents them from speaking frankly to Saudi Arabia about human rights.
"Certainly, international public opinion has turned more towards the reality of what is happening in Saudi Arabia away from the official Saudi promotion of reforms," Adubisi told ABC News. "But is it enough? As a number of states have emphasized in the Human Rights Council, there must be a binding mechanism to hold violators accountable in Saudi Arabia. This may contribute to the protection of dozens of lives."
It may take such radical change, either from within Saudi Arabia or because of pressure from Western nations, to save al-Nimr, al-Zaher, and al-Marhoon.