Security forces have shot dead at least five people and wounded 25 others during the crackdown since Monday's protests
Leading Zimbabwean activist Evan Mawarire was detained by police on Wednesday when he was taken from his house in a widening crackdown after anti-government protests, AFP reporters witnessed.
The nationwide 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.
Police had been at Mawarire's property for more than two hours before he was driven away in a truck.
"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.
"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 the crackdown since Monday's protests, 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.
HRW said security forces responded with live ammunition, rubber bullets and teargas on Monday after protesters burned a police station, barricaded roads with large rocks, and looted shops in Harare, Kadoma and Bulawayo cities.
Mawarire, a pastor, became a prominent voice in 2016 protests when he posted social media videos criticising the government while he wore a Zimbabwean flag around his neck.
The videos inspired the ThisFlag movement that led mass protests across the country against Robert Mugabe, the long-time president who was ousted in 2017 after a military takeover.
Mawarire, who was holding a Zimbabwean flag as he was taken away on Wednesday, has also been a fierce critic of President Emmerson Mnangagwa, a veteran of the ruling ZANU-PF party who succeeded Mugabe.
Mawarire's lawyer Beatrice Mtetwa told reporters he had been taken to a city centre police station.
"They are alleging that he incited violence....through Twitter and other forms of social media particularly in the central business district," she said.
Zimbabwe's mobile phone networks and internet were largely shut down for the second day on Wednesday, while most businesses, offices and shops in the capital Harare have been closed since Monday's protests.
Mnangagwa announced the fuel price hike in a televised address late Saturday, saying it was needed to tackle severe shortages and illegal trading.
Zimbabwe's economy has been in dire straits since hyperinflation wiped out savings between 2007 and 2009, when the Zimbabwean dollar was abandoned in favour of the US dollar.
Mnangagwa -- Mugabe's former deputy -- has claimed that he represents a fresh start and has vowed to revive the shattered economy by attracting foreign investment.
But since his disputed election victory in July, the country has been hit by renewed shortages of fuel, bread, medicine and other daily essentials.