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Soldiers are reported to have taken over the headquarters of Zimbabwe's national broadcaster ZBC, amid a growing political crisis.
Heavy gunfire and artillery have also been heard in northern suburbs, although the situation is unclear, BBC reported on Wednesday.
There has been no word so far from 93-year-old President Robert Mugabe.
Zimbabwe's ambassador in South Africa, Isaac Moyo, dismissed any talk of a coup and said the government was "intact".
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.
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.
Some staff at ZBC were manhandled when soldiers took over their offices in Harare late on Tuesday evening, according to informed sources.
Workers were told that they "should not worry", a source added, and that soldiers were only there to protect the site.
Sounds of heavy gunfire and artillery had been heard in northern suburbs where a number of government officials, including the president, live.
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 US State Department said it was "closely monitoring" the situation in Zimbabwe and urged all parties to resolve disputes "calmly and peacefully".
The UK Foreign Office advised Britons "currently in Harare to remain safely at home or in their accommodation until the situation becomes clearer".
Mugabe sacked Vice-President Emmerson Mnangagwa last week, amid a row over succession.
Mnangagwa had previously been seen as an heir to the president, but First Lady Grace Mugabe is now the clear front-runner.
The rivalry between Mrs Mugabe and Mr Mnangagwa has split Zanu-PF.
Last month, Mugabe warned of a possible coup plot, saying allies of Mnangagwa were threatening the lives of those who didn't support him.
The Zanu-PF party said Gen Chiwenga's comments were "calculated to disturb national peace... (and) incite insurrection".
The party said it would never succumb to military threats, and that it "reaffirms the primacy of politics over the gun".
The leader of Zanu-PF's youth wing, Kudzai Chipanga, said the general did not have the full support of the entire military.
"It is our country and future at stake and we will not let any individual military man interfere with the leader of the party and legitimately voted president of this country," he told reporters on Tuesday.
The youth wing is a strong supporter of Grace Mugabe.
Gen. Chiwenga's warning of possible military intervention came on Monday at a news conference at army headquarters where he was surrounded by senior army officers.
He said the "purging" within Zanu-PF was "clearly targeting members of the party with a liberation background", referring to the country's struggle for freedom from white minority rule.
"We must remind those behind the current treacherous shenanigans that when it comes to matters of protecting our revolution, the military will not hesitate to step in," he said.