A local newspaper reads “Mugabe Under House Arrest” on the second day after the Zimbabwe National Army took over the control of the government, Nov. 16. (Ufumeli/Epa-Efe/Rex/Shutterstock/Ufumeli/Epa-Efe/Rex/Shutterstock)

Zimbabwe's 93-year-old president remained in military custody on Thursday, setting off a frantic effort across southern Africa to resolve the simmering political crisis.

Robert Mugabe, the world's oldest ruling head of state, has been under house arrest since Wednesday morning, although military commanders claimed that the detention did not constitute a coup.

Despite conflicting reports about Mugabe's fate, it appeared his days as president might be numbered, and many Zimbabweans were cautiously optimistic that the country's autocracy was ending.

But the military takeover was likely to be challenged as unconstitutional by both Mugabe and the Southern African Development Community, a regional bloc made up of 15 countries. That would force the Zimbabwean defense forces to either openly defy the constitution or back down from their demands.

"In announcing its intervention, the military is choosing its words very carefully. In part, this is because of governance norms established by the African Union that have taken a hard line against unconstitutional changes of government, particularly those led by the military," noted Monde Muyangwa, director of the Africa Program at the Washington-based Wilson Center.

The African Union's bylaws require it to sanction countries that have undergone a coup and bar them from participating in the organization's activities.

The capital, Harare, remained calm, with brisk and steady traffic even as Mugabe's critics began planning for life after the long-ruling leader, frequently referred to simply as "the old man."

"It's going to be madness," said Victor Matamadanda, secretary general of the Zimbabwe National War Veterans Association, a former ally of Mugabe who like hundreds of other former party stalwarts now opposes the president. "For years, they've treated this country like their own personal village."

The military appears to favor a return of former vice president Emmerson Mnangawa, who was fired by Mugabe last month. Some have suggested that he could lead a transitional government that might include other members of a fragmented opposition.

But first the question of Mugabe's future must be answered. An intelligence source told Reuters that he was resisting the intervention of a Catholic priest encouraging him to step down. A delegation was also dispatched by South African President Jacob Zuma, and a meeting of regional countries was held in Botswana aimed at resolving the crisis.

But the military remained tight-lipped about its plans.

"We can't say what is going to happen," said Overson Mugwisi, a spokesman for Zimbabwe's defense forces.

At around noon, Mugabe's motorcade was seen speeding through Harare, and two military helicopters appeared to take off from the presidential palace. But no one could confirm whether Mugabe had been permitted to leave his home.

The fate of his powerful wife, Grace Mugabe, 52, also remains unknown. It was her apparent plan to succeed him as leader that many believe prompted the military intervention.

Mugabe ruled the country for nearly four decades, leading Zimbabwe — formerly white-ruled Rhodesia — from the triumph of its independence struggle to economic collapse, to finally ending up a prisoner of the military he once commanded.

Mugabe led the country in a guerrilla war that ended white minority rule in 1980. Upon becoming president, he galvanized the population with fiery speeches promising that "Zimbabwe will never be a colony again."

But that mantra lost much of its power in recent years, as Mugabe's presidency was marred by allegations of corruption, nepotism and repression. Zimbabwe went from being one of Africa's wealthiest nations to a country reeling under one of the highest inflation rates in modern history, its currency so devalued that it had to print bank notes of 100 billion Zimbabwean dollars.

Mnangawa was considered one of Mugabe's closest allies. In 2003, the United States sanctioned Mnangawa, labeling him as one of several officials "who undermine democratic processes and institutions in Zimbabwe."

Now, Western officials have expressed concerns that even if Mugabe's possible departure represents a positive step for Zimbabwe, there are lingering worries about the country under Mnangawa.

The military intervention was marked by a televised announcement early Wednesday delivered by a uniformed general as armored personnel carriers dominated the capital's intersections.

Despite the assurances, the events bore all the signs of a coup. Troops were stationed around the city. The army took over the television station. The army spokesman, Maj. Gen. Sibusiso Moyo, said in his televised statement that "criminals" in Mugabe's regime were being targeted.

The commander of Zimbab­we's military forces, Gen. Constantino Chiwenga, made the move as a struggle over who will succeed the country's elderly leader came to a head.

Mugabe recently purged some key officials from the ruling party, ZANU-PF, paving the way for Grace Mugabe to succeed him. Many see that move as a major miscalculation, alienating Mugabe from the civilians and military leaders on whom he had long depended.

The move exacerbated divisions in the ZANU-PF party, where the youth faction is firmly on Grace Mugabe's side, while many older veterans of the struggle against white rule look to Mnangagwa. As a former defense minister, Mnangagwa has strong support in the military.

World leaders have been cautiously monitoring the situation. The British foreign secretary, Boris Johnson, said that "nobody wants simply to see the transition from one unelected tyrant to a next."

The U.S. State Department refrained from calling the action a coup but said Washington was "concerned by recent actions taken by Zimbabwe's military forces" and called on authorities to exercise restraint.

For decades, Mugabe had a reputation as an unwavering critic of many Western policies and international institutions. His supporters hailed him for actions such as the seizure of white-owned farms. Although the farms were meant to be given to black families, many ended up in the hands of Mugabe's close associates, and within years a large number had fallen fallow because their new owners had no background or interest in farming.

Zimbabwe was once a breadbasket for the region, but its economy and especially the agricultural sector have suffered in recent years.

Meanwhile, Mugabe was seen as being increasingly under the influence of his wife, who is also known as "Gucci Grace" for the rumored extravagance of her foreign shopping trips. The country's per capita gross domestic product is $1,008, according to the World Bank.

In recent weeks, there have been signs of an increased sensitivity to criticism of the government. Four people were detained for booing Grace Mugabe at a rally, and an American woman was arrested for allegedly tweeting insulting comments about Mugabe.

Correction: A previous version of this article erroneously described Grace Mugabe's role in the government. She was never installed as one of Zimbabwe's two vice presidents.