Prolonged gunfire erupted near Mr Mugabe's private residence in the suburb of Borrowdale early today, a witness told AFP.
Soldiers quickly seized the state broadcaster, ZBC, manhandled its staff and read out a statement announcing its plans, Reuters reported.
"We want to assure the nation that His Excellency the President of the Republic of Zimbabwe and Commander-in-Chief of the Zimbabwe Defence Forces, Cde R.G. Mugabe and his family are safe and sound and their security is guaranteed", Moyo said.
The US State Department said it was "closely monitoring" the situation in Zimbabwe and urged all parties to resolve disputes "calmly and peacefully".
Mugabe chaired a weekly cabinet meeting in the capital on Tuesday, officials said, and afterwards ZANU-PF said it stood by the "primacy of politics over the gun" and accused Chiwenga of "treasonable conduct. meant to incite insurrection".
"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 added presence of troops as night fell and civilians went home - including the movement of at least six armoured personnel carriers from a barracks north-west of Harare - further sparked rumours of a coup against Mr Mugabe, although there was still no evidence Mr Mugabe had been ousted.
Latest on the Zimbabwe / Grace Mugabe situation " Verified reports confirm that Mugabe has negotiated for Grace to leave the country while he prepares to step down". Grace Mugabe, 52, has support in the party's youth wing and is believed to have engineered the firing of another vice president, Joice Mujuru, in 2014. "They might be trying to give a fig leaf to the notion that President Mugabe is still the leader".
Mugabe abolished the Office of Prime Minister to appoint himself President of Zimbabwe in 1987.
Mugabe - the world's oldest head of state - last week fired Vice President Emmerson Mnangagwa and accused him of plotting to take power, including through witchcraft.
"Defending the revolution and our leader and President is an ideal we live for, and, if need be, is a principle we are prepared to die for", Kudzai Chipanga, Zanu-PF's secretary of youth affairs, told reporters early Tuesday.
But shortly after reports of tanks nearing the capital came through, Zimbabwe's ruling ZANU-PF party released a statement saying it would never succumb to military pressure.
The first lady became head of the ruling party's women's league a few years ago, prompting even more speculation that she'll be her husband's successor. Apparently referring to Mugabe, the tweet said "an elderly man who had been taken advantage of by his wife [was] detained".