Zimbabwean soldiers and armoured vehicles blocked roads to the main government offices, parliament and the courts in central Harare, a Reuters witness said on Wednesday.
While nearby, Zimbabweans queued for cash outside banks as public taxis ferried commuters to work.
"We are only targeting criminals around him (Mugabe) who are committing crimes that are causing social and economic suffering in the country in order to bring them to justice," ZimbabweMajor General SB Moyo, Chief of Staff Logistics, said on national television on Wednesday.
"As soon as we have accomplished our mission, we expect that the situation will return to normalcy."
The military detained Finance Minister Ignatius Chombo on Wednesday, a government source said. Chombo was a leading member of the so-called 'G40' faction of the ruling ZANU-PF party, led by Mugabe's wife Grace, that had been vying to succeed Mugabe.
Soldiers deployed across the Zimbabwe capital Harare on Tuesday and seized the state broadcaster after Mugabe's ruling ZANU-PF party accused the head of the military of treason, prompting frenzied speculation of a coup.
Just 24 hours after military chief General Constantino Chiwenga threatened to intervene to end a purge of his allies in Mugabe's ZANU-PF, a Reuters reporter saw armoured personnel carriers on main roads around the capital.
Aggressive soldiers told passing cars to keep moving through the darkness. "Don't try anything funny. Just go," one barked at Reuters on Harare Drive.
Two hours later, soldiers overran the headquarters of the ZBC, Zimbabwe's state broadcaster and a principal Mugabe mouthpiece, and ordered staff to leave. Several ZBC workers were manhandled, two members of staff and a human rights activist said.
Shortly afterwards, three explosions rocked the centre of the southern African nation's capital, Reuters witnesses said.
Mugabe, the self-styled 'Grand Old Man' of African politics, has led Zimbabwe for the last 37 years.
In contrast to his elevated status on the continent, Mugabe is reviled in the West as a despot whose disastrous handling of the economy and willingness to resort to violence to maintain power destroyed one of Africa's most promising states.
The United States and Britain advised their citizens in Harare to stay indoors because of "political uncertainty."
"U.S. citizens in Zimbabwe are encouraged to shelter in place until further notice," the U.S. statement said. The British Foreign & Commonwealth Office statement told "nationals currently in Harare to remain safely at home or in their accommodation until the situation becomes clearer."
The Southern African nation has been on edge since Monday when Chiwenga, Commander of the Zimbabwe Defence Forces, said he was prepared to "step in" to end a purge of supporters of sacked vice president Emmerson Mnangagwa.
Only a few months ago, Mnangagwa, a former security chief nicknamed "The Crocodile", was favourite to succeed his life-long political patron but was ousted a week ago to pave the way for Mugabe's 52-year-old wife Grace to succeed him.
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