Leading Zimbabwean activist Evan Mawarire was detained by police on Wednesday, AFP reporters witnessed, when he was taken away from his house in the capital Harare.
Armed police had been at Mawarire's property for more than two hours before he was driven away in a pick-up truck as security forces crack down after anti-government protests.
"Armed police surrounded his residence this morning and he has been picked up. We don't know for what purpose," Teldah Mawarire, his sister, told AFP.
"We are not aware of where he has been taken... I was directly in contact with him until he was taken and he could no longer be online. We are very concerned."
Security forces have shot dead at least five people and wounded 25 others during a crackdown since nationwide protests on Monday, according to Human Rights Watch.
"Those responsible for using unlawful lethal force should be promptly investigated and held accountable," Dewa Mavhinga, Southern Africa director at HRW, said in a statement.
It said protesters burned a police station, barricaded roads with large rocks, and looted shops in Harare, Kadoma and Bulawayo cities, while security forces responded with live ammunition, rubber bullets and teargas.
The protests were triggered by the government announcing at the weekend that fuel would more than double in price as the country's economic crisis deepens.