Mr Mawarire, who organised what became nationwide anti-government protests in 2016 against mismanagement and then-president Robert Mugabe's long stay in power, was clutching a Bible when police bundled him into their car.

"They are alleging that he incited violence through Twitter and other forms of social media in the central business district," Mr Mawarire's lawyer Beatrice Mtetwa said.

There were widespread reports of violence as the country faced a third day of protests over what has become the world's most expensive petrol.

This is Zimbabwe's worst unrest since deadly post-election violence in August that saw six people killed.

Zimbabwe's largest telecom company Econet sent text messages to customers saying it had been forced by the government to shut down internet service.

"The matter is beyond our control," it said.

Other arrests were reported, with Nkululeko Sibanda, a spokesman for the main opposition MDC party, saying that "party leadership" had been detained.

"This is only deepening the political crisis in the country," he said.

As President Emmerson Mnangagwa makes an extended overseas trip that will include a stop at the World Economic Forum in Davos, Switzerland, to plead for more foreign investment, former military commander and vice president Constantino Chiwenga, a hardliner, is in charge at home.

Eight people were killed on Monday when police and military fired on crowds, according to Amnesty International.

But Zimbabwe's government said three people were killed, including a policeman who was stoned to death by an angry crowd.

The anti-government demonstrations amounted to "terrorism", information minister Monica Mutsvangwa said on state television.

The protests were "well-co-ordinated" by Zimbabwe's opposition, she said.

The Zimbabwe Association of Doctors for Human Rights said it had attended to 107 patients by late on Tuesday afternoon, with injuries including gunshot wounds to the head.

It said most cases were in Harare and Zimbabwe's second city, Bulawayo.

International concern has been rising over Zimbabwe after a burst of optimism when Mr Mugabe stepped down in late 2017 under military pressure.

The British minister for Africa Harriett Baldwin noted "worrying levels of violence" and urged restraint by Zimbabwe's security forces.

But South Africa's foreign ministry said in a statement that "we're confident measures being taken by the Zimbabwean government will resolve the situation".

Streets were deserted in Harare on Wednesday.

"Shops closed, schools closed, no public transport, petrol stations closed," said Human Rights Watch southern Africa director Dewa Mavhinga.

"Food fast running out in homes," he added.

Zimbabwe's acting president was "silent", he said.

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Police in Zimbabwe armed with AK-47 rifles have arrested activist and pastor Evan Mawarire at his home in Harare as a crackdown grows over protests against dramatic fuel price hikes.