Al-Manassa news website said in a statement that at least eight police raided its offices and searched computers. They seized one laptop, saying they would examine it, the outlet said.
The news outlet said its editor, Nora Younis, was taken in a van to a police station in Cairo’s Maadi district. She faces a charge of managing a news website without an operating license, Al-Manassa said.
The outlet reported that Younis would remain in custody overnight at the police station and appear before prosecutors on Thursday.
Rights group Amnesty International in a tweet urged authorities to "protect her from any torture or other ill-treatment" while in detention.
A spokesman for the Interior Ministry, which oversees police, was not immediately available for comment.
Media are required to have permission to work in Egypt, but withholding accreditation is often used as a pretext to silence reporting the state sees as critical. The outlet has said in previous statements that it applied for a license in October 2018 but has not received a response.
Younis is a rights activist, journalist and blogger. She formerly worked as a managing editor at Al-Masry Al-Youm, Egypt's most popular daily.
Younis founded Al-Manassa in 2015, but authorities blocked the website in 2017, one of hundreds of sites banned or blocked in recent years. The outfit continued to publish through mirror sites, however.
In recent years, Egypt has imprisoned dozens of reporters and occasionally expelled some foreign journalists. It remains among the world’s worst jailers of journalists, along with Turkey and China, according to the Committee to Protect Journalists.
Read also: Egypt detains another journalist on 'fake news' charges
Egypt's government, under general-turned-president Abdel Fattah al-Sisi, has overseen an unprecedented political crackdown, silencing critics and jailing thousands.
Sisi, as defense minister, led the military’s removal of the country’s first democratically elected president in 2013 after his one-year rule proved divisive, sparking massive nationwide protests.
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