Zimbabwe's President Robert Mugabe is being held at his home and is "fine", according to a statement by South Africa's President Jacob Zuma, who has spoken to him.
Mr Mugabe was detained after the military took over control of the country.
Soldiers also seized the headquarters of Zimbabwe's national broadcaster, where loud explosions and gunfire were heard during a night of mounting tension in the capital, Harare.
An army general appeared on television to insist that there had not been a military coup and that the president and his family were "safe and sound".
What has happened?
There are reports of military vehicles blocking roads close to parliament in Harare, and outside the ruling Zanu-PF party headquarters.
Earlier, explosions and gunfire were heard in northern suburbs of the capital, including shots near 93-year-old President Robert Mugabe's private residence.
Troops were said to have entered the headquarters of the national broadcaster ZBC, and Maj Gen Sibusiso Moyo then read out a statement on national television.
He assured the nation that President Mugabe and his family were safe, and insisted his security was guaranteed. The military was only targeting what he called "criminals" around the president, he said, denying that there had been a coup.
On Wednesday morning, a Twitter account which purports to be from Zanu-PF said there had been a "bloodless transition":
Moments before, it claimed that the "first family" had been detained - this claim has not been corroborated.
On Tuesday, Zanu-PF officials accused army chief Gen Constantino Chiwenga of "treasonable conduct" for challenging Mr Mugabe over the sacking of Vice-President Emmerson Mnangagwa last week.
Zanu-PF said Gen Chiwenga's stance was "clearly calculated to disturb national peace... and suggests treasonable conduct on his part as this was meant to incite insurrection".
What hasn't happened?
There has not been any official response from the government to events - which could indicate that the Mugabes are indeed no longer in control, say analysts.
And there are no reports of any members of the security forces who remain loyal to Mr Mugabe intervening to defend him. If they did, events could take a bloodier turn.
Is this a coup?
It's been described as an "extraordinary overnight gamble" by BBC southern Africa correspondent Andrew Harding.
He says it is important to remember that Mr Mugabe is not being challenged by the Western governments he has warned against for decades, or by Zimbabwe's political opposition, or by a mass uprising against economic hardship.
According to our correspondent "this is, fundamentally, an internal power struggle within the governing Zanu-PF party - and whoever emerges victorious can expect a newly purged party to fall, obediently, into line".
In his TV address, Maj Gen Moyo said he wished to "make it clear that this is not a military takeover of government. We want to pacify a degenerating political, social and economic situation in our country".
The military's attempt to downplay the takeover has led to amusing comments on social media, like this one from a renowned Ugandan journalist:
Maj Gen Moyo said the country would return to normality as soon as the military had accomplished what he called its "mission".
What has upset those behind this action?
This is all about the leadership succession, as Mr Mugabe's powers finally falter.
The people who fought in the 1970s guerrilla war against white minority rule still dominate Zimbabwe's government, and especially its security forces, and they are worried about losing that power, and the wealth it generates.
In his statement on Monday, Gen Chiwenga warned against the "purging, which is clearly targeting members of the party with a liberation background".
This was obviously a reference to the sacking last week of Mr Mugabe's once loyal deputy, Mr Mnangagwa, a former defence minister, spy chief and veteran of the war of independence.
He and Grace Mugabe, who is four decades younger than her husband, had been seen as the main candidates to succeed Mr Mugabe.
Mrs Mugabe's supporters are known as "Generation 40" or "G40" - a name which signals a changing of the guard in Zimbabwe, at least partially, 37 years after independence.
So this military action is the old guard reasserting its authority.
Mr Mugabe was the political leader of the guerrilla war so the army has always professed loyalty to him, until he explicitly came out in favour of his wife taking over from him.
What is the mood in the country?
Zimbabweans have been posting on Facebook and Twitter that there has been no dramatic effect on normal life, though some reports say there is an uneasy calm in the capital, Harare.
People say that shops have opened as normal but there are few people on the streets of the capital.
A tweeter posted an update on his experience going to work.
Some reports say there have been mixed emotions, with some people celebrating while others are worried about what will follow.
Reaction across Africa
A statement from South Africa's President Jacob Zuma says that he has spoken to Mr Mugabe who is being held at his home. It says that the veteran leader is "fine".
Mr Zuma had called for "calm and restraint" in an earlier statement.
He said that he was sending a delegation to Zimbabwe to meet President Mugabe and the Zimbabwean Defence Force.
The statement also said that he was sending a team to Angola, in his capacity as the chairman of the Southern African Development Community, to meet President João Lourenço, who heads the body's politics arm.
Nigeria's President Muhammadu Buhari has also called for calm and a "respect for the constitution", and said that every attempt should be made to save the country from "political instability".
The African Union has not yet commented.