After seizing state television, an army spokesman said the group is targeting “criminals” around the the 93-year-old leader.
Neither Mugabe nor his wife Grace, who has been vying to succeed her husband as president, have been seen or heard from, Reuters reports.
There are reports that Mugabe is due to speak at 11am.
“We are only targeting criminals around him (Mugabe) who are committing crimes that are causing social and economic suffering in the country in order to bring them to justice,” Zimbabwe Major General SB Moyo, Chief of Staff Logistics, said on television.
“As soon as we have accomplished our mission, we expect that the situation will return to normalcy.”
Moyo continued: “We wish to make abundantly clear that this is not a military takeover of government. What our defence force is doing is to pacify a degenerate political and social and economic situation in our country, which if not addressed may result in a violent conflict.
“We call upon all the war veterans to play a positive role in assuring peace, stability and unity in the country.”
He added: “To the other security services, we urge you to cooperate for the good of our country, let it be clear that we intent to address the human security threats in our country. Therefore any provocation will be met with an appropriate response.”
The situation has led to an unlikely Twitter spat between two accounts purporting to be the ruling Zimbabwe African National Union Patriotic Front (ZANU-PF) party.
The UK foreign office is advising British nationals currently in Harare to “remain safely at home or in their accommodation until the situation becomes clearer”.
The government’s advice adds: “You should avoid political activity, or activities which could be considered political, including political discussions in public places and criticism of the President. You should avoid all demonstrations and rallies. The authorities have sometimes used force to suppress demonstrations.”
The US embassy has also urged citizens to “shelter in place until further notice”.
Zimbabwe’s opposition Movement for Democratic Change called for a peaceful return to constitutional democracy, adding it hoped the military intervention would lead to the “establishment of a stable, democratic and progressive nation state”.
The leader of Zimbabwe’s influential liberation war veterans called for South Africa, southern Africa and the West to re-engage Zimbabwe, whose economic decline over the past two decades has been a drag on the southern African region.
“This is a correction of a state that was careening off the cliff,” Chris Mutsvangwa told Reuters. “It’s the end of a very painful and sad chapter in the history of a young nation, in which a dictator, as he became old, surrendered his court to a gang of thieves around his wife.”
Finance Minister Ignatius Chombo, a leading member of the so-called ‘G40’ faction of the ruling ZANU-PF party led by Mugabe’s wife Grace, had been detained by the military, a government source said.
Mugabe, the self-styled ‘Grand Old Man’ of African politics, has led Zimbabwe for the last 37 years.
He is the only leader Zimbabwe has known since it gained independence from Britain in 1980
In contrast to his elevated status on the continent, Mugabe is reviled in the West as a despot whose disastrous handling of the economy and willingness to resort to violence to maintain power destroyed one of Africa’s most promising states.
Soldiers deployed across the Zimbabwe capital Harare on Tuesday and seized the state broadcaster after Mugabe’s ruling ZANU-PF party accused the head of the military of treason, prompting frenzied speculation of a coup.
Just 24 hours after military chief General Constantino Chiwenga threatened to intervene to end a purge of his allies in Mugabe’s ZANU-PF, a Reuters reporter saw armored personnel carriers on main roads around the capital.
Aggressive soldiers told passing cars to keep moving through the darkness. “Don’t try anything funny. Just go,” one barked at Reuters on Harare Drive.
Two hours later, soldiers overran the headquarters of the ZBC, Zimbabwe’s state broadcaster and a principal Mugabe mouthpiece, and ordered staff to leave. Several ZBC workers were manhandled, two members of staff and a human rights activist said.
Shortly afterwards, three explosions rocked the centre of the southern African nation’s capital, Reuters witnesses said.
The United States and Britain advised their citizens in Harare to stay indoors because of “political uncertainty.”
The southern African nation had been on edge since Monday when Chiwenga, Commander of theZimbabwe Defence Forces, said he was prepared to “step in” to end a purge of supporters of sacked vice president Emmerson Mnangagwa.
Only a few months ago, Mnangagwa, a former security chief nicknamed “The Crocodile”, was favorite to succeed his life-long political patron but was ousted a week ago to pave the way for Mugabe’s 52-year-old wife Grace to succeed him.