Border officers stopped Loujain Hathloul when she tried to drive from neighbouring United Arab Emirates into Saudi Arabia on Sunday. Maysaa Alamoudi, a UAE-based Saudi journalist, later arrived to support her.
An activist told AFP on Wednesday that Hathloul, 25, and Alamoudi were still in custody in Eastern Province but neither had been charged.
The activist declined to be named.
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"Given that both women appear to be detained because they were driving, Saudi officials should immediately release them and end the discriminatory driving ban on women," said HRW.
The New York-based watchdog's Middle East and North Africa director Sarah Leah Whitson said: "After years of false promises to end its absurd restrictions on women, Saudi authorities are still arresting them just for getting behind the wheel."
"The Saudi government's degrading restrictions on women are what bring shame to the country, not the brave activists standing up for their rights," she said.
Before her arrest on Monday, Hathloul wrote on Twitter that she had been stuck at the frontier for 24 hours waiting to cross from the UAE to Saudi Arabia.
The interior ministry has so far not commented on the case, and referred enquiries to the customs department
During October, dozens of women drove in the kingdom and posted images of themselves doing so as part of an online campaign supporting the right to drive.
In response, the interior ministry said it would "strictly implement" measures against anyone undermining "the social cohesion".
Activists say it is not actually against the law for women to drive and that the ban is linked to tradition and custom in the kingdom.
