Zimbabwe’s military has read out a statement on the country’s national broadcaster, ZBC, saying it has taken action to “target criminals”.
However, it said this was not “a military takeover of government” and President Robert Mugabe was safe.
Heavy gunfire and artillery were heard in northern suburbs of the capital, Harare, early on Wednesday.
Zimbabwe’s envoy to South Africa, Isaac Moyo, earlier dismissed talk of a coup, saying the government was “intact”.
A statement read out by the military on TV said the army wanted to deal with people who “were committing crimes that are causing social and economic suffering in the country.
“As soon as we have accomplished our mission, we expect that the situation will return to normal.”
The statement said 93-year-old President Mugabe and his family were safe and their security was guaranteed.
The UK Foreign Office advised Britons “currently in Harare to remain safely at home or in their accommodation until the situation becomes clearer”.
The US embassy in Harare tweeted that it would be closed on Wednesday “due to ongoing uncertainty”.
It also advised US citizens in Zimbabwe to “shelter in place” until further notice.
The worsening situation came after Zimbabwe’s ruling party accused the country’s army chief of “treasonable conduct” after he warned of possible military intervention.
General Constantino Chiwenga had challenged President Mugabe after he sacked the vice-president.
Gen Chiwenga said the army was prepared to act to end purges within Mr Mugabe’s Zanu-PF party.
Tensions were raised further on Tuesday when armoured vehicles were seen taking up positions on roads outside Harare, although their purpose was unclear.