Military tanks were on Tuesday spotted heading toward Zimbabwe's capital a day after the military chief warned President Robert Mugabe over purge in the ruling Zanu-PF party.
Witnesses told said military vehicles were also blocking major roads outside Harare.
A Reuters witness saw two other tanks parked beside the main road from Harare to Chinhoyi, about 20 kilometres from the city.
One, which was pointed in the direction of the capital, had come off its tracks.
The troops, believed to be from Inkomo Barracks on the outskirts of Harare, were heavily armed.
Local media reported that soldiers had sealed off state TV broadcaster — the Zimbabwe Broadcasting Corporation.
The Nation's correspondent in the capital said anxiety had gripped Zimbabwe following an "unusual movement" of troops.
Witnesses told AFP that several tanks had been spotted on the outskirts of the capital.
"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.
Zimbabweans were thrown into panic after a flurry of pictures of the military artillery appeared in social media.
There were no reports of violence but the situation in the capital was tense.
The movements by soldiers occurred hours after Zanu-PF youth leaders told journalists in Harare that they were ready to die for President Mugabe following army commander Constantino Chiwenga's unprecedented warning.
The reason for the military presence was not immediately clear but the vehicles may have been on routine manoeuvres.
The military spokesman and Defence Minister Sidney Sekeremayi were not available for comment.
The development comes a day after the country’s head of the armed forces, General Constantino Chiwenga, said he was prepared to “step in” to end a purge of supporters of ousted Vice-President Emmerson Mnangagwa.
General Chiwenga demanded a "stop" to the purge in the ruling Zanu-PF party after the sacking of Mr Mnangagwa, and warned the military could intervene.
"The current purging which is clearly targeting members of the party with a liberation background must stop forthwith," General Chiwenga told a media conference attended by about 90 senior army officers at army HQ.
In an unprecedented warning, he said in a statement: "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."
Mr Mnangagwa fled into exile in South Africa after he was pushed out of the party but vowed to fight Mr Mugabe and his wife Grace, the likely successor of the the 93-year-old president.
Analysts had warned that the sacking would spark repercussions beyond Mr Mugabe's control.
Mr Mnangagwa's main rivals within the ruling Zanu-PF- party are the younger "Generation 40" or "G40" group, who enjoy Grace's support.
But the 75-year-old former vice president has powerful military connections, having served as defence and state security minister.
The majority of Zimbabwe’s army commanders are veterans of the country’s liberation war and were heavily involved in the ruling party politics.
President Mugabe has in the past complained about the military’s meddling in politics and his wife Grace recently claimed some commanders were threatening a coup if Mr Mnangagwa was not allowed to succeed her husband.
Mr Mugabe, the world's oldest president, is showing increasing signs of old age, but has refused to name his successor.
Zimbabwe goes to elections next year to vote for a president and lawmakers.
Additional reporting by AFP and agencies.