Journalism watchdog calls on Saudi Arabia to release blogger
Nafjan was arrested with six other activists associated with the women's rights movements, known particularly for their work against a driving ban on women in the country.
According to an AP report, Nafjan and the others detained have not been allowed access to lawyers and their whereabouts are unknown.
"Crown Prince Mohammad Bin Salman recently toured the West to project the image of a modernist and reformer. But the moment he's back home Saudi authorities revert to old habits - stifling dissent and detaining critical journalists," CPJ Deputy Executive Director Robert Mahoney said.
"The crown prince needs to translate rhetoric into action and end the repression of journalists in the kingdom. Writing about the place of women in Saudi society is not a crime. Eman al-Nafjan must be released immediately."
The advocates were arrested on bizarre list of charges including, "transgressing national and religious principle" and working with "foreign entities" ahead of a driving ban on women set to be lifted.
Women in Saudi Arabia will be allowed to start driving in the ultra-conservative kingdom from 24 June.
While some have hailed the "progressive move" it has also been accompanied by an apparent crackdown on dissent against critics, ranging from Islamist clerics to some of the very women who campaigned for years to end the ban.
Activists and analysts believe that the government is keen to avoid rewarding activism.
"Women active within the movement were threatened by the Saudi state security apparatus last year after the impending end of the ban was announced and were ordered not to speak to international media, according to the Gulf Centre for Human Rights and the Project on Middle East Democracy," the CPJ highlight said in their statement.
Saudi Arabia has described the activists as "traitors" and has left public opinion divided.
Referring to the detainees as "agents of the embassies", political analyst Naif al-Asaker tweeted that anyone who did not support their arrest was either a "covert partner" or simply "ignorant".
Amnesty International condemned the commentary of the arrests as a "chilling smear campaign" and an "extremely worrying development for women human rights defenders" in the country.
In September, a royal decree announced the end of a decades' long ban on women driving - the only one of its kind in the world.
Crown Prince Mohammed bin Salman, 32, is seen as the force behind the lifting of the ban, part of a series of reforms being pushed by the powerful royal in the conservative kingdom.
His economic Vision 2030 reform plan for a post-oil era seeks to elevate women to nearly one-third of the workforce, up from about 22 percent now.
Saudi activists say social change will only be cosmetic without dismantling the kingdom's rigid guardianship system, which requires women to seek permission from a male relative to study, travel and other activities.