The UN has expressed concern over the continuing and “apparently arbitrary” crackdown on Saudi human rights activists after two more prominent female campaigners were arrested in the kingdom.

Samar Badawi, an internationally recognised activist, and Nassima al-Sadah, a co-founding member of Al-Adalah Center for Human Rights, were detained earlier this week.

At least 15 prominent activists have been held as part of a government campaign that began in the run-up to the much publicised lifting of the ban on women driving. Many other cases are thought to remain unreported.

Khalid Ibrahim, executive director of the Gulf Centre for Human Rights, said the crackdown was unprecedented.

 “When you describe the human rights situation in other countries you say the space for civil society is shrinking. In Saudi Arabia there is no civic space left to shrink. They are putting every peaceful voice behind bars,” he said. “If you are a human rights defender you will be treated worse than a criminal.”

Eight people arrested this summer have been temporarily released pending a procedural review, but face the threat of rearrest. Some activists are to be tried in the specialized criminal court, established to hear terrorism cases, where they could be sentenced to up to 20 years in prison.

The UN’s human rights office warned of a “serious lack of transparency” in the handling of the cases. Activists in detention have had no access to their families or lawyers, said Ibrahim.

Badawi, a recipient of the International Women of Courage award, was a prominent figure in the call to end the driving ban for women, a landmark reform passed in June and credited to Saudi prince Mohammed bin Salman.

Al-Sadah was barred from standing as a candidate in local elections in 2015, the first year in which women were allowed to run.

Both al-Sadah and Badawi had challenged the country’s male guardianship system, which requires women to obtain permission from their fathers, brothers, husbands or even sons for a range of basic life decisions.

On Monday, Amal al-Harbi, the wife of prominent Saudi activist Fowzan al-Harbi, was also arrested. Her husband is a founding member of the Saudi Civil and Political Rights Association, a now banned civil society group, and is serving a seven-year sentence for his activism.

The Regional Coalition for Women Human Rights Defenders in the Middle East and North Africa said the country’s recent reforms were designed only to improve its international reputation, not to improve women’s rights.

“Saudi Arabia has responded to the world’s call to free jailed women human rights defenders and women’s rights activists by arresting more,” said the coalition’s Islam al-Khatib.

“Reforms that are tightly managed and limited from the top are hardly reforms at all.”

More than 30 human rights groups have warned of a growing climate of fear among female campaigners in both Saudi Arabia and Egypt.

In a recent open letter to the UN special rapporteur on the situation of human rights defenders, the groups wrote: “Saudi authorities, government-aligned media, and troll accounts on social media launched a public smear campaign and labelled women human rights defenders as ‘traitors’ and a ‘danger to Saudi society and national security.’”

Badawi has been harassed by the Saudi authorities for years, having previously been barred from travelling abroad in 2014, and briefly detained over her human rights advocacy in 2016. She has campaigned for equality and for her former husband and her brother to be released from prison.

Waleed Abu al-Khair, her former husband, is serving a 15-year sentence for his human rights work, while Raif Badawi, her brother, is serving a 10-year sentence for expressing controversial opinions online.

“Allies and partners considering opportunities for closer ties with Saudi Arabia during this period of ‘reform’ should speak out against Mohammad bin Salman’s ultimately self-defeating repression,” said Sarah Leah Whitson, Middle East director for Human Rights Watch.

“Any economic vision that seeks to open up Saudi Arabia while throwing real reformers in jail may well end badly for everyone.”