Human Rights Watch said Tuesday it was "highly unlikely" that 81 men Saudi Arabia executed in a single day received a fair trial, calling it a "brutal show of its autocratic rule".
More than half of them, 41, belonged to the Shiite minority "who have long suffered systemic discrimination and violence by the government", the rights group said.
The executions on Saturday were the most announced in a single day in Saudi Arabia, one of the world's most prolific users of the death penalty, and outstripped its entire total last year.
"Saudi Arabia's mass execution of 81 men this weekend was a brutal show of its autocratic rule, and a justice system that puts the fairness of their trials and sentencing into serious doubt," HRW's deputy Middle East director Michael Page said in a statement.
"The shocking callousness of their treatment is compounded by the fact that many families found out about their loved ones' deaths just like the rest of us, after the fact and through the media."
Saudi Arabia said the men, who included seven Yemenis and a Syrian, were convicted of crimes related to "terrorism" and belonged to the Islamic State group, Al-Qaeda, Yemen's Huthi rebels or "other terrorist organisations".
Their trials were "supervised by a total of 13 judges in three separate stages", official media said.
But HRW said: "Rampant and systemic abuses in Saudi Arabia's criminal justice system suggest it is highly unlikely that any of the men received a fair trial.
"Many Saudi (Shiites) are serving lengthy sentences, are on death row or have been executed for protest-related charges following patently unfair trials," it added.
HRW said it had obtained court judgements for five of the Shiites, finding due process violations in all of them.
"In every case they had told the court that they suffered torture and ill-treatment during interrogations, and that their confessions were forcibly extracted."
The report quoted a brother of one of the 81 who said he had only found out about the executions through media.
"We have no idea how and what time they were killed, how and where they were buried," he was quoted as saying.
"I keep wondering, what were my brother's last words? Was he buried according to (Shiite) burial rites? Did they pray over his body?"
The executions were widely condemned, with UN human rights chief Michelle Bachelet raising concerns over potential breaches of international law and even war crimes.
