Heavy gunfire and artillery have also been heard in northern suburbs, although the situation is unclear.
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 a possible military intervention.
General Constantino Chiwenga had challenged President Robert Mugabe after he sacked the vice-president.
Gen 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, sources told Reuters.
Workers were told that they “should not worry”, a source added, and that soldiers were only there to protect the site.
The BBC’s Shingai Nyoka, in Harare, said the sounds of heavy gunfire and artillery had been heard in northern suburbs where a number of government officials, including the president, live.
The US State Department said it was “closely monitoring” the situation in Zimbabwe and urged all parties to resolve disputes “calmly and peacefully”.
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.
Mugabe sacked Vice-President Emmerson Mnangagwa last week, amid a row over succession.
Mr 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, Mrs Mugabe warned of a possible coup plot, saying allies of Mr Mnangagwa were threatening the lives of those who didn’t support him. (Courtesy BBC)