Amnesty International and Human Rights Watch accused Saudi Arabia on Tuesday of subjecting several activists, including some female human rights defenders, detained since May to torture and sexual harassment.
Over a dozen women's rights activists have been arrested since May, most of whom had campaigned for the right to drive and an end to the kingdom's male guardianship system, though some have since been freed.
A group of UN experts last month called for the immediate release of six female human rights defenders it said were still being held incommunicado in the kingdom.
The May arrests followed an earlier crackdown on clerics, intellectuals, and activists in September 2017 in an apparent bid to silence potential opponents of Saudi Arabia's de facto ruler, Crown Prince Mohammed bin Salman.
The torture allegations come with Saudi Arabia facing an international outcry over the killing last month of Saudi journalist Jamal Khashoggi in the Saudi consulate in Istanbul.
In a statement, London-based Amnesty said that according to testimony from three individuals it had gathered, some of the detained activists "were repeatedly tortured by electrocution and flogging" that left some "unable to walk or stand properly".
In a similar statement, Human Rights Watch quoted "informed sources" as saying Saudi interrogators tortured at least three female Saudi activists.
Both Amnesty and Human Rights Watch said the detainees were subjected to sexual harassment.
"Only a few weeks after the ruthless killing of Jamal Khashoggi, these shocking reports of torture, sexual harassment and other forms of ill-treatment, if verified, expose further outrageous human rights violations by the Saudi authorities,” Lynn Maalouf, Amnesty International's Middle East research director, said in the statement.
Reuters could not verify the allegations and the rights groups did not clarify the identity of the individuals who provided testimony. Human Rights Watch said the sources were concerned that they and the activists would suffer reprisals if identified publicly.
A Saudi government communications office did not immediately respond to a request for comment.
Riyadh has in the past denied using torture and said that arrests were made on the basis of suspicious contacts with foreign entities and offering financial support to "enemies overseas."
Last June the Saudi government ended a decades-old ban on women driving cars as part of a bid to diversify the economy away from oil and open up Saudis' cloistered lifestyles.
But while many hailed the end of the ban on women driving as proof of a new progressive trend, it has been accompanied by a crackdown on dissent.
Turkish President Tayyip Erdogan has said the killing of Khashoggi was ordered at the "highest levels" of the Saudi government but has not directly accused Prince Mohammed.
Saudi Arabia has denied that the prince ordered Khashoggi's killing.