HARARE, Zimbabwe — Emmerson Mnangagwa, who fled into a brief exile after losing a power struggle less than three weeks ago, became Zimbabwe’s new president Friday — succeeding Robert Mugabe, the leader he had backed for decades before helping oust him last week.

“We should never remain hostages of our past,’’ Mnangagwa said in his inaugural address. Referring specifically to Mugabe, he said: “To me personally, he remains a father, mentor, comrade in arms, and my leader.’’

In a rapid reversal of fortunes that abruptly ended one of the longest reigns in Africa’s postcolonial history, Mnangagwa was sworn in at a ceremony at the National Sports Stadium in Harare, the capital, attended by about 60,000 people.

Mnangagwa, 75, who served for half a century as his predecessor’s right-hand man, became only the second leader of Zimbabwe since independence in 1980.

Under an alternately sunny and cloudy sky, Mnangagwa said in his address that the country’s domestic politics had “become poisoned and rancorous and polarizing,’’ apparently referring to the factional fighting inside the governing party, ZANU-PF.

He promised that democratic elections will be held on schedule in 2018 and that foreign investment will be safe in Zimbabwe, laying the groundwork for a program to revive the economy and end the country’s chronic severe unemployment.

Mnangagwa added that his compatriots should “let bygones be bygones, readily embracing each other in defining a new destiny in our beloved Zimbabwe.’’

He emphasized rebuilding the country’s economy by, in part, reengaging with Western nations that cut off most ties with Zimbabwe after the seizure of white-owned farms starting in 2000.

Mnangagwa said that compensation would be offered to those who had lost their properties, signaling his commitment to a process that had begun fitfully in recent years.

Those in the stadium, many of them hard-core political supporters, wildly cheered Mnangagwa and hailed him as a “hero’’ and “liberator.’’ But even as he promised a new era of democracy, the incoming president, who was long known as Mugabe’s ruthless enforcer, faced a far more skeptical nation.

Mugabe, 93, who remains under house arrest with his wife, did not attend the event. George Charamba, Mugabe’s spokesman, said that Mnangagwa had extended an invitation to the former president, but that Mugabe would stay home because “emotions and feelings are running high.’’

Even before the euphoria over the end of the Mugabe era began to subside, many opposition politicians, rights activists, ordinary citizens, and even some party members were expressing concerns about entrusting a new Zimbabwe to a leader so closely tied to the old.

“This is a happy day,’’ said Virginia Kamoto, 34, a ZANU-PF member who was bused in with other supporters from the Masvingo area in southern Zimbabwe.

“I was personally tired of Mugabe, who had stayed for far too long in power,’’ she added. “I hope President Mnangagwa will not overstay in power. I hope he will not repress the people or tolerate corruption so that our country will be counted among the great nations of the world.’’

For some, the genuine and deep excitement over seeing Mugabe go was already fading before the reality that Mnangagwa and the rest of the old guard were still firmly in place.

After learning of Mugabe’s resignation on Tuesday, Mevion Gambiza, 28, said he had celebrated with others outside the hotel where lawmakers had gathered to start impeaching the former president.

Gambiza then rode on the roof of a taxi as its driver honked through the streets of Harare. Gambiza, a sociology graduate from the University of Zimbabwe, said he came to the new president’s inauguration because of its historical significance.

“Nothing will change; poverty and suffering will continue,’’ he said. “It’s just another ZANU-PF faction that has outcompeted its rival and now Mnangagwa’s bootlickers will have their full turn to loot from the state coffers.’’

But Emerson Zinyera, 54, a retired police officer who described himself as a strong backer of the new president, said: “Today is true independence day. The one that was there was false. Today is independence that everyone, every Zimbabwean can enjoy, not independence enjoyed by two people, Mugabe and his wife, Grace.’’

Mnangagwa’s exact role in the military intervention that led to Mugabe’s downfall is not yet known.

But on Wednesday, just hours after returning to Zimbabwe from South Africa, Mnangagwa thanked the generals who had backed him, saying he had been “in constant contact with the service chiefs throughout’’ the recent events.

The victory of Mnangagwa and the military — over a ZANU-PF faction led by Mugabe’s 52-year-old wife, Grace, and younger politicians with no experience in the war of liberation — underscored the old guard’s enduring grip on power, not only in Zimbabwe but also in nations like Angola, Mozambique, Namibia, and South Africa.