Denied the right to travel without consent from their male guardians and banned from driving, women in Saudi Arabia are now monitored by an electronic system that tracks any cross-border movements.
Since last week, Saudi women's male guardians began receiving text messages on their phones informing them when women under their custody leave the country, even if they are travelling together.
Manal al-Sherif, who became the symbol of a campaign launched last year urging Saudi women to defy a driving ban, began spreading the information on Twitter, after she was alerted by a couple.
The husband, who was travelling with his wife, received a text message from the immigration authorities informing him that his wife had left the international airport in Riyadh.
"The authorities are using technology to monitor women," said columnist Badriya al-Bishr, who criticised the "state of slavery under which women are held" in the ultra-conservative kingdom.
Women are not allowed to leave the kingdom without permission from their male guardian, who must give his consent by signing what is known as the "yellow sheet" at the airport or border.