The United Nations human rights office has called on Saudi Arabia to release all peaceful activists as Riyadh continues its heavy-handed crackdown on dissent.

Ravina Shamdasani, spokeswoman of the UN rights office, during a press briefing in the Swiss city of Geneva on Tuesday demanded the immediate release of all rights advocates, including women held for campaigning to lift a long-time ban on driving.

"We urge the government of Saudi Arabia to unconditionally release all human rights defenders and activists who have been detained for their peaceful human rights work, including their decades-long campaigns for the lifting of the driving ban for women," Shamdasani said.

“Dissent, criticism of the government is still not accepted in the country. That can explain why many of these human rights defenders and activists have been jailed. All of them have criticized government policies in one way or another,” she added.

According to the rights office, at least 15 government critics have been arrested since mid-May, the whereabouts of some are unknown amid a serious lack of transparency in the processing of their cases.

Prominent women's rights advocate Hatoon al-Fassi was one of the 15 people. She was arrested in June as she was planning to take journalists in her car to celebrate the removal of the ban on female drivers.

In July, Saudi Arabia took into custody a prominent Muslim scholar, Sheikh Safar al-Hawali, known for his opposition to some of the kingdom’s policies and the presence of American troops on the Saudi soil.

International human rights groups have long urged the kingdom to improve its treatment of rights advocates. In early June, the European Parliament issued a resolution calling for their unconditional release.

Most of the detainees are prominent figures who enjoy considerable respect among the grass roots, including university professors.

Saudi authorities have so far labeled the detainees “traitors”, infuriating the country’s rights activists who fear additional arrests amid much-hyped reports of reforms led by Saudi Crown Prince Mohammed bin Salman.