Zimbabwe's military has read out a statement after taking over the national broadcaster, ZBC, saying it has taken action to "target criminals".
However, it said this was not "a military takeover of government" and President Robert Mugabe was safe.
Heavy gunfire and artillery were heard in northern suburbs of the capital, Harare, early on Wednesday.
Zimbabwe's envoy to South Africa, Isaac Moyo, earlier dismissed talk of a coup, saying the government was "intact".
The statement read out by Maj Gen Sibusiso Moyo came hours after soldiers overran the headquarters of ZBC.
He said: "We wish to assure the nation that his excellency the president... and his family are safe and sound and their security is guaranteed."
The statement added: "We are only targeting criminals around him who are committing crimes... that are causing social and economic suffering in the country. As soon as we have accomplished our mission, we expect that the situation will return to normalcy."
The statement did not name those targeted but a government source quoted by Reuters said Finance Minister Ignatius Chombo was among those detained.
It is not clear who is leading the military action.
Armoured vehicles were seen taking up positions on roads outside Harare on Tuesday
Other key points of the statement included:
- Citizens should remain calm and limit unnecessary movement
- The military assures the Zimbabwean judiciary that its independence is guaranteed
- Security services should "co-operate for the good of our country" and any provocation would "be met with an appropriate response"
- All leave for the defence forces is cancelled and they should return to barracks immediately
The UK Foreign Office advised Britons "currently in Harare to remain safely at home or in their accommodation until the situation becomes clearer".
The US embassy in Harare tweeted that it would be closed on Wednesday "due to ongoing uncertainty".
It also advised US citizens in Zimbabwe to "shelter in place" until further notice.
The latest events came hours after Zimbabwe's ruling party accused the country's army chief of "treasonable conduct" after he warned of possible military intervention.
General Constantino Chiwenga had challenged 93-year-old President Mugabe after he sacked the vice-president.