HARARE, Zimbabwe — President Robert Mugabe, who has ruthlessly ruled Zimbabwe for nearly four decades and was one of Africa’s last liberation leaders, was under house arrest on Wednesday, hours after the military announced it had taken him into custody in what appeared to be a coup.

The fate of Mr. Mugabe, 93, who kept a tight grip on his southern African nation despite his increasing frailty and diplomatic isolation from the West, appeared to be in the hands of former allies and opposition officials negotiating his future.

In the capital, Harare, about half a dozen tanks were stationed around strategic government buildings and intersections. But shops and banks were open, and most people carried on business as usual. Soldiers blocked the main road leading to the airport, which Mr. Mugabe, 93, had renamed after himself just last week.

The military did not say whether Mr. Mugabe had been removed as president, leaving the possibility that he may be kept on during a period of transition. But whatever happens to Mr. Mugabe, it appeared increasingly clear on Wednesday that an era was coming to a close in Africa.

Power appeared to be slipping from Mr. Mugabe as outside forces were determining his fate.

President Jacob Zuma of South Africa said that he had spoken on the phone with Mr. Mugabe on Wednesday morning, and that the Zimbabwean leader had said he was fine but confined to his house in Harare.

Emmerson Mnangagwa — the vice president who was ousted by Mr. Mugabe last week in a power grab by allies of Grace Mugabe, 52, the president’s wife — was reported to be on his way back to Zimbabwe and was widely seen as the country’s new real leader.

Negotiations were underway between Mr. Mnangagwa’s allies and opposition parties to possibly form an interim government of unity that would soften international criticism of the military takeover.

No resistance could be seen from forces that had long remained loyal to the president, including the presidential guard and the vast network of secret intelligence that had helped Mr. Mugabe keep a grip on the nation despite a crumbling economy and diplomatic isolation.

A soldier mounting a military tank at a major intersection said, “We are here to put things right in the country.”

Mr. Mnangagwa, 75, was considered close to military leaders, including the commander of the Zimbabwe Defense Forces, Gen. Constantine Chiwenga, who had threatened military intervention early this week.

Though a veteran of the ruling ZANU-PF party, Mr. Mnangagwa was also known to be on good terms with Morgan Tsvangirai, the longtime leader of the main opposition party, the Movement for Democratic Change.

“We just changed the head of the train,” Chris Mutsvangwa, a close ally of Mr. Mnangagwa’s who is also the leader of the war veterans association and a former minister under Mr. Mugabe, said in a phone interview.

Mr. Mutsvangwa said that, in a possible interim government, Mr. Mnangagwa would serve as president and Mr. Tsvangirai as prime minister.

“It’s more of a coalition rather than a coup,” Mr. Mutsvangwa said. “Mugabe was senile and incapacitated, and things were being run behind the scenes by a small coterie and his wife.”

But opposition officials said no decision had been made and that they remained wary of entering into any alliance with Mr. Mnangagwa.

“We have to make sure we aren’t being sold a dummy,” Elias Mudzuri, a deputy president of the Movement for Democratic Change, said by phone. “We aren’t sure of the endgame. They say this is not a coup, but, to us, it’s a coup.”

An aide to Mr. Tsvangirai — who has been in South Africa for medical treatment — said that the opposition leader was on his way back to Zimbabwe on Wednesday.

Experts said that efforts to form a unity government were an attempt to blunt criticism of the coup and win the support of foreign governments and international creditors at a time when Zimbabwe’s coffers are empty.

“Mnangagwa’s biggest challenge is legitimacy,” said Dewa Mavhinga, a researcher on Zimbabwe for Human Rights Watch, the advocacy group, by phone from Harare. “That’s why he is likely to try to accommodate Tsvangirai.”

Early Wednesday, after the military seized the state broadcaster, ZBC, two uniformed officers read a short statement in which they denied carrying out a coup.

They also said that they had taken Mr. Mugabe in custody to protect him and that they were “only targeting criminals around him who are committing crimes that are causing social and economic suffering in the country.”

The message appeared intended to provide a palatable exit for Mr. Mugabe, or to keep him on during a period of transition. Despite his long troubled rule and increasingly poor health, Mr. Mugabe, as one of Africa’s last liberation leaders, commanded respect inside the ruling party and on the continent.

On Wednesday, Mr. Mnangagwa’s allies reinforced their longtime message that Mr. Mugabe had been led astray by his wife and other members of a faction called the G-40. Leading figures of that faction were reportedly arrested in Harare on Wednesday.

Since entering politics a couple of years ago, Mr. Mugabe’s wife has accumulated power by maneuvering aside the presumed successors to her husband, including Joice Mujuru, a former vice president. But she and her allies appeared to have overreached in orchestrating the removal of Mr. Mnangagwa last week.

Mr. Mnangagwa, whose nickname is the Crocodile, has been the leader of a rival faction called Team Lacoste. People close to him in Mr. Mugabe’s government were considered moderates open to reconciliation with Western governments and international creditors. Zimbabwe became politically isolated after Mr. Mugabe ordered the invasion of white-owned farms in the early 2000s.

But Mr. Mnangagwa has a longstanding reputation as a sometimes brutal ally of Mr. Mugabe. He is believed to have played a key role in the Gukurahundi massacres of the Ndebele ethnic group in the 1980s.

“He represents to many of us the very pathology of anti-democratic tendencies in ZANU-PF,” said Ibbo Mandaza, an academic and businessman active in opposition politics. “There is a naïve expectation that anybody other than Robert Mugabe will be better. But a military coup is bad news.”

In central Harare, business went on as usual on Wednesday. Shops were open and long lines formed outside banks, which have been low on cash for two years. Some people openly praised the military while many others said they feared speaking amid the uncertainty.

Tendai Muganhu, 43, a street vendor, said, “I am happy because I know whoever will come into power won’t be like Mugabe, won’t chase vendors from streets, but will certainly improve our lives.”