Saudi authorities detained seven women’s rights defenders since May 15, Human Rights Watch says.

Saudi Arabia has arrested a number of female rights activists in the kingdom over the past days, rights group Human Rights Watch (HRW) and the Gulf Centre for Human Rights said.

In a statement issued on Saturday, US-based HWR said Saudi authorities had detained seven women’s rights defenders since May 15.

The women have long been advocating an end to the ban on Saudi women driving and the abolishment of the male guardianship system, the group said.

“Crown Prince Mohammad bin Salman’s ‘reform campaign’ has been a frenzy of fear for genuine Saudi reformers who dare to advocate publicly for human rights or women’s empowerment,” HRW Middle East Director, Sarah Leah Whitson, said in a statement.

“The message is clear that anyone expressing skepticism about the crown prince’s rights agenda faces time in jail.”

Among the detained women are Eman al-Nafjan, a Saudi blogger, and Lujain al-Hathloul, a women’s rights activist who had been arrested previously and held for 75 days for attempting to drive back into Saudi Arabia from neighbouring United Arab Emirates (UAE).

According to testimonies given to HRW, the women claimed the royal court had ordered them not to speak to the media last year when the decision to reverse the long-standing ban on women driving was announced via royal decree.

As of June 24, women will be allowed to drive for the first time in the kingdom, “in accordance with the Islamic laws”.

On Friday, the Gulf Centre for Human Rights also said that “several human rights defenders were arrested”.

According to group, al-Hathloul has not been able to communicate with her family or lawyer since being arrested, while al-Nafjan managed to contact her family once thus far.

Saudi Arabia has some of the world’s tightest restrictions on women, despite ambitious government reforms aimed at boosting female employment.

Rothna Begum, a researcher at HRW, said the government is trying to silence critics, particularly those who champion women’s rights reforms.

“While it’s not clear why they were arrested, today we have seen Saudi press reports come to suggest that these women are traitors and have been arrested because they are undermining the national unity of the country,” Begum told Al Jazeera.

“What we know is that the Saudi crown prince wants to make it clear to all of his citizens that they are his subjects who must be grateful for whatever liberties he gives them, but they must not demand any of their rights.”

The country’s 32-year-old crown prince has widely been credited with being behind the kingdom’s lifting of the driving ban, part of a wider set of reforms being implemented as part of the Vision 2030 plan.

However, the country’s guardianship system remains in place, under which a male family member – usually the father, husband or brother – must grant permission for a woman’s study, travel and other activities.

Saudi activists claim that the guardianship issue is at the core of the fight for women’s rights.

Since 2011, nearly 30 activists and dissidents have been convicted in Saudi courts, many of whom received sentences of up to 15 years, according to HRW.