Human Rights Watch (HRW) voiced concern for dozens of Palestinians and Jordanians on trial in Saudi Arabia, calling on the kingdom to "immediately make clear the specific accusations and underlying evidence against defendants".
In a report released on Friday, the rights group cited relatives as saying that the detainees, who were arrested over the past two years, had been abused in Saudi prisons.
Saudi Arabia began the trials of 68 Palestinians and Jordanians last month, accusing them of links to an unidentified "terrorist organisation" that is believed to be Hamas.
Sources told Middle East Eye at the time that the charges against the accused ranged from the bizarre to the indefensible.
One person is accused of possessing bottles of Palestinian olive oil, a second is being charged with sending sheep to people in Gaza for the Muslim holiday of Eid al-Adha, and a third is accused of possessing a book on the history of Palestine by Kuwaiti writer Tareq al-Suwaidan, the sources said.
According to HRW, the suspects have not been presented with "specific accusations or evidence".
"Saudi Arabia's long record of unfair trails raises the specter that Jordanians and Palestinians will be railroaded on serious charges and face severe penalties even though some have alleged serious abuses," said Michael Page, HRW's deputy Middle East director.
"At a time when Covid-19 presents acute dangers to prisoners, Saudi Arabia should consider alternatives to detention, particularly for those in pretrial detention."
The Palestinian group Hamas has repeatedly denounced the mass trials, calling them "unjust".
"The Palestinians arrested by the Saudi state security police have committed no crimes other than having the honour of defending Jerusalem and al-Aqsa mosque," the Palestinian group said in statement in March.
HRW interviewed relatives of seven defendants, who said the suspects had been missing for months, with Saudi authorities refusing to disclose their whereabouts.
"Family members of defendants described a range of abuses by Saudi authorities after the arrests, including enforced disappearances, long-term solitary confinement, and torture," the rights group said.
Early in April, Amnesty International called for the release of former Hamas representative in the kingdom, Mohammed al-Khudari, 81, and his son, saying that they were being prosecuted "in a mass trial on trumped-up charges under the counter-terror law".
"They have no legal representation. Dr Mohammed al-Khudari requires adequate medical attention and treatment for cancer," Amnesty said in a statement.
On Friday, HRW cited Khudari's case to convey fears by the suspects' families over their loved-ones' well-being amid the coronavirus outbreak.
"Families of defendants in the current mass trial expressed serious concerns about the possible outbreak of Covid-19 in Saudi prisons and are calling for their release," HRW said in a statement.
Yemen's Houthi rebels, who are locked in a military conflict against Saudi Arabia, offered last month to release five Saudi prisoners in their custody in exchange for freeing the Palestinian and Jordanian detainees in the kingdom.
Hamas welcomed the Houthi proposal, which appears to have gone unanswered by Riyadh.