Zimbabwean military officers read an address live on state TV in the early hours of Wednesday, saying they were not launching a coup but were "targeting criminals around" President Robert Mugabe.
"It is not a military takeover of government," said one general reading a statement.
"We wish to assure the nation that his excellency the president... and his family are safe and sound and their security is guaranteed.
"We are only targeting criminals around him who are committing crimes... As soon as we have accomplished our mission we expect that the situation will return to normalcy."
Armoured vehicles were seen on the streets near the capital Harare on Tuesday as questions mounted over whether Mugabe, who has ruled since independence from Britain in 1980, was still holding onto power.
Tensions between the 93-year-old leader and the military have intensified in recent days.
Prolonged gunfire erupted near Mugabe's private residence in the suburb of Borrowdale early Wednesday, a witness told AFP. No further details were available.
The shooting came after Mugabe's ZANU-PF party on Tuesday accused army chief General Constantino Chiwenga of "treasonable conduct".
The public dispute has presented a major test for Mugabe, who is in increasingly frail health.
Chiwenga had demanded that Mugabe stop purges of senior party figures, including vice president Emmerson Mnangagwa, who was dismissed last week.
Mugabe under pressure
Mugabe is the world's oldest head of state, but his frail health has fuelled a bitter succession battle as potential replacements jockey for position.
Some of the army top brass are seen as strongly opposed to Grace Mugabe's apparent emergence as the likely next president.
"We very rarely see tanks on the roads," Derek Matyszak, an analyst at the Pretoria-based Institute for Security Studies, told AFP.
"Chiwenga threw down the gauntlet to Mugabe... it would make sense for Chiwenga to organise some military manoeuvres to up the ante.
"It's clear we are entering new territory here."
In speeches this year, Mugabe has often slurred his words, mumbled and paused for long periods.
His lengthy rule has been marked by brutal repression of dissent, mass emigration, vote-rigging and economic collapse since land reforms in 2000.
The main opposition MDC party called for civilian rule to be protected.
"No one wants to see a coup... If the army takes over that will be undesirable. It will bring democracy to a halt," shadow defence minister Gift Chimanikire, told AFP.
ZANU-PF's influential youth league, which supports Grace Mugabe as the next president, said in a statement that army chief Chiwenga must not be allowed to choose Zimbabwe's leaders.
Speculation has been rife in Harare that Mugabe could seek to remove Chiwenga, who is seen as an ally of ousted Mnangagwa.
The crisis marks an "ominous moment in the ongoing race to succeed" Mugabe, said political analyst Alex Magaisa in an online article.
"(Mugabe) has previously warned the military to stay away from ZANU-PF's succession race.
"His authority over the military has never been tested in this way."
Mnangagwa, 75, was widely viewed as Mugabe's most loyal lieutenant, having worked alongside him for decades.
He fled the country and is thought to be in South Africa after issuing a searing five-page condemnation of Grace's ambition and Mugabe's leadership.
Earlier this year the country was gripped by a bizarre spat between Grace and Mnangagwa that included an alleged ice-cream poisoning incident that laid bare the pair's rivalry.
Mnangagwa took over as vice president from Joice Mujuru who was axed in 2014 after Grace Mugabe launched a campaign accusing her of plotting to topple the president.
Grace Mugabe - 41 years younger than her husband -- has become increasingly active in public life in what many say is a process to help her eventually take the top job.