The UN Secretary-General Antonio Guterres has appealed for calm, non-violence and restraint
The head of the African Union has warned that the takeover of Zimbabwe by the country's military "seems like a coup."
United Nations Secretary-General Antonio Guterres appealed for calm, non-violence and restraint after gunfire and explosions were heard near Mr Mugabe's compound.
The army chiefs who seized the capital, including the presidential palace and the state broadcaster, early on Wednesday have claimed they have not carried out a coup.
President Robert Mugabe is believed to be under house arrest despite an army claim that he was "safe."
The Zimbabwean army said it took Mr Mugabe and his wife Grace into custody but there are unconfirmed and disputed reports that the first lady has fled the country and may be in Namibia.
Armoured vehicles blocked roads to the main government offices, parliament and the courts in central Harare, and the military said the action was aimed at targeting "criminals."
Army spokesman Major General Sibusiso Moyo said Mr Mugabe remains in power:
"We are only targeting criminals around him who are committing crimes that are causing social and economic suffering in the country, in order to bring them to justice," he said.
The Irish embassy in Pretoria is advising Irish citizens to remain indoors, and to avoid areas where demonstrations may be taking place.
Due to the political uncertainty in Zimbabwe at this time we advise all Irish citizens to remain indoors and to avoid areas where demonstrations may be taking place. Please monitor this account for updates.— IrishEmbassyPretoria (@IrlEmbPretoria) November 15, 2017
African Union leader Alpha Conde, who is also Guinea's president, said the AU condemned the actions of military chiefs in the southern African country, adding that they were "clearly soldiers trying to take power by force."
"The African Union expresses its serious concern regarding the situation unfolding in Zimbabwe," he said, before demanding "constitutional order... be restored immediately" as he called "on all stakeholders to show responsibility and restraint."
South African President Jacob Zuma said he had spoken to Mr Mugabe over the phone, adding that the 93-year-old "is fine but confined in his home."
Mr Zuma has also sent an envoy to speak to Zimbabwean army chiefs.
Blasts could be heard in the capital as the military took control of a paramilitary police armoury and government offices.
The political crisis was sparked by Mr Mugabe's sacking of vice president - and likely successor - Emmerson Mnangagwa last week.
The 75-year-old former intelligence chief, whose ousting appeared to clear the way for Mrs Mugabe to succeed her husband, was accused of plotting against the government.
Following his dismissal, Mr Mnangagwa fled to South Africa and called on members of ruling party ZANU-PF to desert the President.