It was not clear whether the apparent military coup would bring a formal end to the 93-year-old Mugabe's rule. The main goal of the generals appeared to be preventing Mugabe's wife Grace, 41 years his junior, from succeeding him.
Local media reported South Africa's defence and state security ministers, dispatched by President Jacob Zuma as regional envoys, arrived in Zimbabwe's capital, Harare, on Wednesday night and were expected to meet both Mugabe and the military. Their ultimate goal was not clear.
The South African presidency said Mugabe had told Zuma over the phone that he was confined to his home but was otherwise fine and the military said it was keeping him and his family safe.
He plunged Zimbabwe into a fresh political crisis last week by firing his vice president and presumed successor, Emmerson Mnangagwa, 75 - known as the "crocodile" - for showing "traits of disloyalty".
Tanks blocked roads after dark and soldiers with automatic weapons kept up their patrols, but the situation appeared calm.
Whatever the final outcome, the events could signal a once-in-a-generation change for the former British colony, a regional breadbasket reduced to destitution by economic policies Mugabe's critics have long blamed on him.