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 Mugabewho 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.
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".