Several tanks were seen moving near the Zimbabwean capital Harare on Tuesday, witnesses told AFP, a day after the army warned it could intervene over a purge of ruling party officials.
The sightings came as uncertainty swirls in Harare due to President Robert Mugabe's decision to fire his vice president last week, prompting a warning of possible military intervention from the army chief.
The reason for the military presence was not immediately clear, but the vehicles may have been on routine manoeuvres.
The military spokesman was not available to comment.
"I saw a long convoy of military vehicles, including tanks, about an hour ago. I don't know where they were heading," a female fruit seller near Westgate shopping centre, about 10 kilometres (six miles) from central Harare, told AFP.
A second female by-stander at the shopping centre also told the AFP reporter that she had seen the convoy, while other witnesses took to social media to confirm the reports.
Zimbabwe's army chief General Constantino Chiwenga on Monday warned Mugabe to "stop" purges of the ruling ZANU-PF party after Mugabe abruptly sacked vice president Emmerson Mnangagwa last week.
"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 at a press conference.
Mnangagwa had clashed repeatedly with First Lady Grace Mugabe, 52, who is widely seen as vying with Mnangagwa to succeed her husband as president.
- 'Upping the ante'? -
Mugabe, 93, is the world's oldest head of state and has ruled since independence from Britain in 1980, but his frail health has fuelled succession talk as potential replacements jockey for position.
"We very rarely see tanks on the roads. They don't normally move around very much," Derek Matyszak, an analyst at the Pretoria-based Institute for Security Studies, told AFP.
"Chiwenga threw down the gauntlet to Mugabe... Mugabe hasn't responded immediately but it would make sense for Chiwenga to organise some military manoeuvres to up the ante."
In speeches this year, Mugabe has often slurred his words, mumbled and paused for lengthy periods.
His long reign 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 on Tuesday 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 a future 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.
- Mugabe tested -
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. If he does nothing, it might be regarded as a sign of weakness. If he puts his foot down, it could result in open confrontation."
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 but has yet to make a public appearance 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, 52, 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.
© 2017 AFP