Saudi Arabian women's rights activist Loujain al-Hathloul has been detained by authorities at an airport in Dammam, rights group Amnesty International said on Monday.
This is the second time authorities have detained Hathloul, who was arrested on Sunday, Amnesty said.
In 2014, she was held for breaking an absolute ban in the Gulf kingdom on women driving - the only country in the world where women are not allowed to do so.
"It appears she is being targeted once again because of her peaceful work as a human rights defender speaking out for women's rights, which are consistently trammelled in the kingdom," said Amnesty's Samah Hadid.
"If so she must be immediately and unconditionally released," said Hadid, who described Monday's news as "absurd and unjustifiable".
In November 2014, Hathloul tried to drive into Saudi Arabia from the neighbouring UAE in defiance of the ban, and was held for more than two months.
At the time, campaigners for Hathloul's release did not provide full details of the allegations against her but said investigations appeared to focus on social media use rather than driving.
Saudi citizens are highly active on social media including on Twitter, where many post anonymously under fabricated user names.
However, Hathloul's account, which has 284,000 followers, is run under her real identity.
Hathloul tried to stand for public office in November 2015, in the kingdom's first election in which women were allowed to vote and stand for office.
The activist said she was disqualified by the authorities.
Amnesty has said Hathloul was later recognised as a candidate, but her name was never added to the ballot.
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