Egypt's security forces arrested well-known blogger and activist Wael Abbas in the early hours of Wednesday, his sister Rasha Abbas told CNN.
Security forces stormed Abbas' house in the Egyptian capital's district of New Cairo, confiscating his laptop, phone and books as well as a toy gun, according to his sister.
His latest post on Facebook read: "I'm being arrested." His sister said that the family has no information about his whereabouts.
Egyptian authorities did not immediately respond to CNN's request for comment.
Abbas is a high-profile blogger who has publicly advocated for a democratic transition in Egypt since the early 2000s. He was a leading voice in the Arab Spring protests of 2011 that saw the ouster of former President Hosni Mubarak. Abbas has also exposed cases of torture and has been critical of a continuing crackdown on dissent.
Thousands have been arrested since a popularly supported military coup in 2013 deposed former President Mohammed Morsi and brought Abdel Fattah el-Sisi to power one year later.
Hours before Abbas was arrested, a military court sentenced researcher Ismail al-Iskandarani to 10 years in prison for "obtaining and publishing military secrets, publishing false news and joining a banned organization," his lawyer Tarek Abdel Aal told CNN on Wednesday.
Iskandarani has been in detention since 2015. He is a leading researcher and journalist on the restive Sinai Peninsula and has helped to shed light on the humanitarian situation there.
The Egyptian government has cut off access to the Sinai as it battles and Islamist insurgency that includes a local ISIS affiliate. Thousands of civilians there are reported to have been caught in the crossfire and subjected to forced displacement, according to Human Rights Watch.
Sisi told France 24 last October that there were no political detainees in Egypt and that "there are normal trial proceedings that are fair."
In Iskandarani's case, Abdel Aal described an opaque judicial process. "The prosecution didn't explain what secrets he divulged. They only presented a number of articles and TV interviews to the court without explaining what's at fault in them," he said.
Both Islamist and secular activists have been arrested in the Egyptian government's crackdown on dissent. Some have been subjected to torture and solitary confinement for extended periods of times, according to multiple reports by rights groups, including Amnesty International and Human Rights Watch.
Egyptian authorities consistently deny the use of the torture.
In its most recent annual report, Amnesty International gave a scathing assessment of the country for its crackdown on dissent in recent years.
Despite the clampdown on civil liberties, Sisi has throngs of supporters who argue that many of his stern measures have been necessary to bring order to Egypt after the chaos of the 2011 uprising. His re-election this year saw him receive 97 percent of the vote, according to Egypt's National Election Authority. But most of his high-profile opponents in that election were arrested or reportedly intimidated out of the race.