A Saudi woman who tried to drive into the kingdom in defiance of a ban was arrested Monday after being blocked at the UAE border for a day, activists said.
Conservative Saudi Arabia is the only country in the world which does not allow women to drive.
"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," Loujain Hathloul said in a Tweet at around midday, before Tweets from @LoujainHathloul stopped.
Activists said she was arrested at the border with the United Arab Emirates on Monday afternoon, but the interior ministry could not immediately comment on her case.
Another woman, UAE-based Saudi journalist Maysaa Alamoudi, who went to support her, was also arrested, an activist told AFP.
"They transferred her and Maysaa... to the bureau of investigation" at a Saudi police station, said the activist who asked for anonymity.
Both women's phones rang unanswered.
Border officers blocked Hathloul because she was driving, and asked her to wait until they received "orders from their superiors," activists said.
"The customs (department) have no right to prevent me from entering even if in their opinion I am 'a violator' because I am Saudi," Hathloul tweeted on Monday morning.
She said her driving licence "is valid in all GCC countries", a reference to the six-nation Gulf Cooperation Council which includes Saudi Arabia.
Hathloul also posted details about her long confinement in her car.
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She said Alamoudi arrived "and has brought me a toothbrush. Refreshing".
"Thank God I still have some petrol left," she wrote.
"If it weren't for that I would have died of the cold last night and my phone switched off. Yaala, I am almost 20 hours now at the border."
Six hours into her wait she had said she was "optimistic", and earlier joked: "If someone brings me a horse or a camel to the border, maybe then I'll be allowed to pass."
The activist who spoke to AFP said Hathloul was trying to make a point in her unusual attempt to drive through the border.
"She knew that they wouldn't let her pass," the activist said.
The October 26 Driving Campaign, a group of activists, said in a statement that Hathloul and Alamoudi "did not in any way break the law of Saudi Arabia".
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".
Women drivers have previously been arrested and cars have been confiscated, activists say.
They say women's driving is not actually illegal, and the ban is linked to tradition and custom in the Islamic nation.