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.
Latest reaction and updates
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.
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”.