The military has taken control in Zimbabwe but said President Robert Mugabe, in power since 1980, was safe.
After seizing state TV, an army spokesman announced it was targeting people close to Mr Mugabe.
South Africa’s President Jacob Zuma later said he had spoken to Mr Mugabe who had indicated that he “was confined to his home but said that he was fine”.
The move may be a bid to replace Mr Mugabe with his sacked deputy, Emmerson Mnangagwa, BBC correspondents say.
The dismissal of Mr Mnangagwa last week had left Mr Mugabe’s wife Grace as the president’s likely successor.
Heavy gun and artillery fire could be heard in northern parts of the capital Harare early on Wednesday.
A statement read out by a general on air denied it was a coup. There was no immediate word from the president himself.
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The rise of Grace Mugabe
Mr Mugabe, 93, has dominated the impoverished country’s political scene since independence from the UK.
Mr Zuma earlier said he hoped events in Zimbabwe would not lead to “unconstitutional changes of government”.
Messages appeared on a Twitter account purportedly run by the ruling Zanu-PF party saying Mr Mugabe had been detained. But there has been no confirmation by the army and it is not clear who runs the account.
The UK Foreign Office advised Britons “currently in Harare to remain safely at home or in their accommodation until the situation becomes clearer”, while the US embassy in Harare advised US citizens in Zimbabwe to “shelter in place” until further notice.
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China, Zimbabwe’s biggest trading partner, says it is closely watching the situation and hopes that the relevant parties can properly handle their internal affairs.
How did the military justify its move?
Troops in armoured vehicles have been out in the streets of the capital Harare since Tuesday.
After soldiers overran the headquarters of the ZBC broadcaster, Maj Gen Sibusiso Moyo went on air to say the military wished to “assure the nation that his Excellency the president… and his family are safe and sound and their security is guaranteed”.
Image copyright Reuters
Image caption Soldiers patrolled Harare on Wednesday
“We are only targeting criminals around him who are committing crimes… that are causing social and economic suffering in the country,” he said.
“As soon as we have accomplished our mission, we expect that the situation will return to normalcy.”
Other key points of the statement include:
Citizens should remain calm and limit unnecessary movement
The military assures the Zimbabwean judiciary that its independence is guaranteed
Security services should “co-operate for the good of our country” and any provocation would “be met with an appropriate response”
All leave for the defence forces is cancelled and personnel should return to barracks immediately
It is not clear who is leading the military action.
Army chief Gen Constantino Chiwenga, who visited China last week, said on Monday the army was prepared to act to end purges within Zanu-PF.
The old guard strikes back
By Joseph Winter, BBC News website Africa editor
This is all about the leadership succession, as Mr Mugabe’s powers finally falter.
The people who fought in the 1970s guerrilla war against white minority rule still dominate Zimbabwe’s government and especially its security forces, and they are worried about losing that power and the wealth it generates.
In his statement on Monday, Gen Chiwenga warned against the “purging, which is clearly targeting members of the party with a liberation background”.
This was obviously a reference to the sacking last week of Mr Mugabe’s once loyal deputy, Mr Mnangagwa, a former defence minister, spy chief and veteran of the war of independence.
He and Grace Mugabe, who is four decades younger than her husband, had been seen as the main candidates to succeed Mr Mugabe. Mrs Mugabe’s supporters are known as Generation 40 or G40 – a name which signals a changing of the guard in Zimbabwe, at least partially, 37 years after independence.
So this military action is the old guard reasserting its authority. Mr Mugabe was the political leader of the guerrilla war so the army always professed loyalty to him until he explicitly came out in favour of his wife.