Concerned relatives have not been told where Loujain al-Hathloul is being held or even if she has been charged

The family of a Saudi woman being held in custody for attempting to drive her car, in defiance of the country’s ban on female drivers, has been told that her detention has been extended by 25 days.

Worried relatives and friends of Loujain al-Hathloul, 25, who has been in custody in Saudi Arabia since 1 December, are yet to be told what charges she faces. They fear that authorities may intend to make an example of her, after becoming frustrated by low-level campaigning by activists against the ban. To date few of the women who have flouted the ban have been held for more than a few days.

Hathloul, driving from the United Arab Emirates, was deliberately defying the law when she was blocked and forced to stay in her car at the border crossing with Saudi Arabia for a day. The French literature graduate had taken part in an online campaign against the ban last year, posting a video of herself driving home from Riyadh airport.

On Saturday, a group of six women who drove their cars in the east of the country in support of her cause, were all arrested.

Hathloul also made waves on Saudi social networks several months ago by posting a video of herself with her face and hair uncovered, in defiance of her country’s ultra-conservatives.

At that point her father was admonished by the ministry of the interior, which forced him to sign a pledge not to allow his daughter to drive again. Her uncle was also asked to release a statement denouncing her actions.

Hathloul’s campaign has brought her thousands of online admirers. Among them is Eman al-Nafjan, author of the popular Saudiwoman blog. “I don’t care how cliched it is, she really is like a breath of fresh cool air in this hot desert climate,” Nafjan said.

Another woman, UAE-based Saudi journalist Maysaa Alamoudi, who supports Hathloul, had also been arrested, a friend said, saying that Alamoudi had not intended to cross on to Saudi soil but had been tricked into doing so and then arrested by armed guards and taken into custody along with her friend.

“Maysaa just came to the border to help and support Loujain and to bring her food, when she heard that she had been trapped there with her passport confiscated,” said the friend, who has requested anonymity because of threats the family had received. Hathloul’s father and brother have also been detained.

“We think they have been taken to different prisons and have been interrogated several times,” said the friend. “But any more than this we do not know. The authorities will tell us nothing, their phones are not answered and we are very worried about the conditions they are being kept in and how they are coping.

“We have no clue to what they might be planning and we are very worried. Hathloul is a very brave woman, but in this case she will be very scared, very shocked,” the source said.

“Her family, her father especially, are trying to be strong, but they are concerned.”

Activists said border officers stopped Hathloul as soon as they saw her at the wheel and asked her to wait until they received orders from their superiors.

Hathloul posted details about her long confinement in her car, taking pictures of the low temperature readings overnight and of her supplies of water. Her driving licence “is valid in all GCC countries”, a reference to the six-nation Gulf Cooperation Council which includes Saudi Arabia.

Six hours into her wait she said she was “optimistic”. “If someone brings me a horse or a camel to the border, maybe then I’ll be allowed to pass,” she joked. Then: “The customs [department] have no right to prevent me from entering, even if in their opinion I am ‘a violator’ because I am Saudi.”

Her last tweet was: “I have been at the Saudi border for 24 hours. They don’t want to give me my passport nor will they let me pass.” When her tweets stopped, users and activists took to Twitter to express concern and offer support. “She has had a lot of support, but there are also a lot of people attacking her. It’s noticeable that those attacking her seem to be using pseudonyms in accounts opened just for this purpose,” said the friend.

During October, dozens of women drove in Saudi Arabia and posted images of themselves doing so as part of an online campaign supporting the right to drive. The interior ministry warned them it would strictly implement measures against anyone undermining what it described as the social cohesion of the kingdom.

Over the past few years several women drivers have been arrested and their cars confiscated, activists say. They include Manal al-Sharif, who helped start the women’s right to drive campaign in Saudi Arabia in 2011 and was jailed for nine days. She was only released after being forced to sign a bail condition that she wouldn’t drive again. “This sentence on Loujain is unprecedented,” said one activist, “so of course we are very fearful of what it might mean.”

The Saudi interior ministry would not comment on the case.