Soldiers stormed the headquarters of Zimbabwe's ZBC state broadcaster in the early hours of Wednesday, two members of staff and a human rights worker said, compounding speculation of a coup against 93-year-old President Robert Mugabe.
Prolonged gunfire also erupted near Mugabe's private residence in the suburb of Borrowdale, a witness told AFP.
Zimbabwe Defence Forces Major General SB Moyo makes an announcement on state broadcaster ZBC, in this still image taken from a November 15, 2017 video.
He assured the nation that President Mugabe and his family were safe, and insisted his security was guaranteed.
"As soon as we accomplish our mission we expect situation to return to normalcy", Moyo said.
But the Zanu-PF party said Chiwenga's comments were meant to "incite insurrection" and that it "reaffirms the primacy of politics over the gun" and accused him of "treasonable conduct".
Tanks were seen roaming moving through the streets as troops entered buildings in the capital Harare.
A USA embassy spokesman in Zimbabwe said the embassy would be minimally staffed and closed to the public on Wednesday.
Chiwenga's unprecedented statement represented a major escalation of the struggle to succeed Mugabe, the only leader Zimbabwe has known since it gained independence from Britain in 1980.
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The US State Department is encouraging Americans in the country to "shelter in place until further notice" due to reports of violence and ongoing political unrest, while the UK Foreign Office said it is "monitoring the situation closely" and advised people to avoid demonstrations and rallies.
Mnangagwa had previously been seen as Mugabe's successor but was replaced by the president's wife, Grace Mugabe, as the favorite, resulting in a split in the Zanu-PF.
Chombo was a leading member of the "G40" group in the ruling Zanu-PF party, led by Mugabe's wife Grace. Grace Mugabe has developed a strong following in the powerful youth wing.
Mnangagwa, 75, was widely viewed as Mugabe's most loyal lieutenant, having worked alongside him for decades.
Zimbabwe's Commander of Defense Forces General Constantino Chiwenga said on Monday he was prepared to "step in" to end a purge of supporters of the sacked vice president.
Zimbabwe's ruling party has claimed on social media that President Robert Mugabe and his family were detained Tuesday night in what increasingly looks like an attempt to depose longtime president.
Grace Mugabe's rise has brought her into conflict with the independence-era war veterans, who once enjoyed a privileged role in the ruling party under Mugabe, but who have in recent years been banished from senior government and party roles.
In an address to the nation, an army spokesman said the military is targeting "criminals" around Mugabe, and sought to reassure the country that order will be restored.
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