Saudi Arabia has reportedly arrested three more women's rights activists in a crackdown launched just weeks before a ban on women driving will be lifted.
Human rights groups said at least 11 people, most of them women who had long campaigned for the right to drive, had now been detained since last week.
Officials have said they are suspected of "suspicious contact with foreign parties" and undermining "stability".
Other activists have said the crackdown is "unprecedented" and "shocking".
The US has expressed concern about the detentions and said it is "keeping a close eye" on the progress of reforms in the Gulf state, which is a key regional ally.
King Salman bin Abdulaziz Al Saud and his son, Crown Prince Mohammed bin Salman, drew widespread praise last year when they announced that the decades-old ban on women driving would end on 24 June.
Saudi women's rights activists, including those who have been imprisoned for defying the ban, celebrated the decision. But they also vowed to continue campaigning for the end of other laws they consider discriminatory.
Women must adhere to a strict dress code, be separated from unrelated men, and be accompanied by or receive written permission from a male guardian - usually a father, husband or brother - if they want to travel, work or access healthcare.
During an interview with CBS News in March before a trip to the US, the crown prince stated: "Saudi women still have not received their full rights. There are rights stipulated in Islam that they still don't have. We have come a very long way and have a short way to go."
Okaz newspaper with a full-page feature of arrested #Saudi women’s rights activists. Again, brands them as “traitors” with “no place among us”. It states that sources predict sentences up to 20 years for 6 violations of Royal Decree A.44. (first pic via @SarahDadouch). pic.twitter.com/eXfWiBVZPt— Nora Abdulkarim نورة الدعيجي (@Ana3rabeya) May 20, 2018
But on Friday, it emerged that the Saudi authorities had recently detained seven prominent activists - five women and two men. A statement issued the following day by the Presidency of the State Security - which reports directly to the king's office - said they faced charges for "suspicious contact with foreign parties" and undermining the "security and stability" of the state.
Pro-government newspapers and social media accounts branded them "traitors".
Human Rights Watch said the detainees included women's rights advocates Loujain al-Hathloul, Aziza al-Yousef, and Eman al-Nafjan, along with Mohammed al-Rabea, an activist, and Ibrahim al-Modaimeegh, a human rights lawyer.
A source stating that @LoujainHathloul @azizayousef @Saudiwoman are in solitary detention in Dhahban prison, North of Jeddah, along with another woman activist of unconfirmed identity yet, prison administration is preparing for a visit of HR entities https://t.co/LKPlJbaEdr— هالة الدوسري (@Hala_Aldosari) May 22, 2018
On Wednesday, HRW cited Saudi activists as saying at least another four other women's rights defenders were now being held, bringing the total number to at least 11.
The Associated Press meanwhile cited people familiar with the arrests as saying the detained activists had not been given access to lawyers, and that they were allowed just one telephone call to their families a week ago. One of the women is reportedly being held entirely incommunicado.
Activists also told AP that seven of those detained had recently submitted a request to the government to establish a non-governmental organisation called "Amina" that would offer support and shelter to victims of domestic abuse.
"The crown prince, who has styled himself as a reformer with Western allies and investors, should be thanking the activists for their contributions to the Saudi women's rights movement," said Sarah Leah Whitson, HRW's Middle East director.
"Instead, the Saudi authorities appear to be punishing these women's rights champions for promoting a goal [Mohammed] bin Salman alleges to support - ending discrimination against women."
The 32-year-old crown prince has also overseen a crackdown on influential clerics and intellectuals since being named heir to the throne last June.
He has also spearheaded a sweeping anti-corruption drive which resulted in dozens of princes, government ministers and businessmen being detained in November and generated an estimated $107bn ($80bn) in settlements.