Without naming those detained, Saudi Arabia's state security apparatus said seven people had been arrested for "attempting to undermine the security and stability of the kingdom... and to erode national unity".
"Work is still underway to identify everyone involved" and take legal measures against them, according to a security spokesman quoted Saturday by state news agency SPA.
Those arrested are facing accusations including making "suspicious contact with foreign parties", providing financial support to "hostile elements abroad" and recruiting government workers.
The crackdown comes even as the kingdom breaks with long-held restrictions on women and the mixing of the genders, with its driving ban on women slated to end June 24.
But there were warnings that Riyadh would not tolerate those pushing for change outside its authority.
Activists told Human Rights Watch that in September 2017, on the same day authorities announced the driving ban would be lifted, the Royal Court had called up prominent activists and warned them not to speak to the media.
'Chilling smear campaign'
With a front page reading "Your betrayals have failed", Al-Jazirah newspaper named two of those arrested as activists Loujain al-Hathloul and Aziza al-Yousef.
On Twitter -- a popular tool of communication for Saudi Arabia's young population -- opinions were sharply divided.
"No place for traitors among us," SaudiNews50 wrote.
The news website's post also carried images of five of those detained: Hathloul, Yousef, Eman al-Nafjan, Mohammed al-Rabiya and lawyer Ibrahim al-Madmyegh.
Supporters of Crown Prince Mohammed bin Salman, seen as the architect behind a succession of social reforms emanating from Riyadh, backed the sweep.
Referring to the detainees as "agents of the embassies", political analyst Naif al-Asaker tweeted that anyone who did not support their arrest was either a "covert partner" or simply "ignorant".
Amnesty International condemned the commentary of the arrests as a "chilling smear campaign" and an "extremely worrying development for women human rights defenders" in the country.
For other Saudis, the treatment of trailblazers who fought for many of the reforms now coming to fruition was a shock.
"This round of arrests was strictly targeting Saudi feminism," tweeted Saudi-American activist Nora Abdulkarim.
Abdullah al-Aoudh, a Saudi-educated scholar now at Yale Law, said the charges did not add up.
"For those who consider these arrests to be in defence of religion and religious scholars, I'd just like to remind you that the scholars and intellectuals of the country are in jail... in the same scenario," he tweeted.
Reform 'frenzy of fear'
The detainees had not only fought for the lifting of the driving ban, but also against the kingdom's enduring guardianship laws.
Saudi Arabia's guardianship system requires women to obtain permission from their fathers, brothers, husbands or even sons for a host of life decisions.
Activists Hathloul and Nafjan in 2016 signed a petition to abolish Saudi Arabia's male guardianship system, according to HRW.
Madmyegh, a semi-retired lawyer, has stepped up to represent fellow rights advocates in recent years.
"Crown Prince Mohammed bin Salman's 'reform campaign' has been a frenzy of fear for genuine Saudi reformers," said Sarah Leah Whitson, Middle East director at HRW.
Many Saudi activists say social change will only be cosmetic without dismantling the kingdom's guardianship system.
