Zimbabweans read morning newspapers Wednesday, a day after President Robert Mugabe resigned. (Ben Curtis/AP)

The man expected to lead Zimbabwe after the dramatic toppling of longtime president Robert Mugabe returned from abroad on Wednesday, promising democracy but also warning that the ruling party would remain firmly in control.

Emmerson Mnangagwa's arrival in Harare marked the beginning of an era that many here were already referring to as a rebirth. After 37 years of Mugabe's increasingly authoritarian rule, the nation of 16 million was jubilant at the prospect of more freedom and an improvement in the fragile economy.

But Zimbabwe is entering a deeply uncertain period. Its new leader is a man with a dubious legacy, who was appointed through a shadowy, closed-door process. He faces immense challenges in resurrecting an economy that shrank during decades of political turbulence and is burdened by $11 billion in debt.

Mnangagwa is a longtime Mugabe ally, nicknamed "the crocodile" due to a reputation for savviness and a willingness to use harsh tactics.

He is expected to be appointed president within days, becoming the country's second leader since independence from Britain in 1980. Mnangagwa was dismissed several weeks ago by Mugabe, precipitating a military takeover that on Tuesday led to the longtime president's resignation. Mnangagwa then left the country due to concerns over his safety.

On his return Wednesday, he delivered a short, ebullient speech in front of a roaring crowd at the headquarters of the ruling party, ZANU-PF, which has nominated him as the country's next leader.

"Today we are witnessing the beginning of a new unfolding democracy in our country," he said, after flying in from South Africa.

"I pledge myself to be your servant," Mnangagwa added. "I appeal to all genuine parties in Zimbabwe to come together and work together. No one is more important than the other. We are all Zimbabweans."

But switching from English to the local Shona language, Mnangagwa took a darker turn, belittling the opposition.

"Those who oppose us will bark and bark," he said. "They will continue to bark, but the ZANU-PF train will roll on, ruling and ruling while they bark."

Many Zimbabweans have questioned the legality of his ascent — Mnangagwa had no formal position in government after being fired, and he is not in the constitutional line of succession. But the ruling party, which Mugabe ran for nearly four decades before it abandoned him last week, claimed that it had the authority to appoint the next president.

Mnangagwa arrives in power at a time of extraordinary unity in Zimbabwe.

For the first time in decades, white farmers, activists, soldiers and opposition groups marched together through the streets of Harare this past week in defiance of Mugabe. Many people have concerns about Mnangagwa but have largely accepted the inevitability of his leadership and are hoping for the best.

Mnangagwa and Mugabe were part of the cadre of liberation leaders who fought to oust the white-minority government when the country was known as Rhodesia. For his role in the movement, Mnangagwa served 10 years in Harare Prison, including some time with Mugabe.

When Mugabe was elected prime minister, Mnangagwa also began to climb the ranks of government. Opposition politicians and human rights activists have accused him of helping to engineer a crackdown on the Ndebele tribe, whose members were seen to be against the Mugabe regime in the early 1980s. The operation resulted in the murder of roughly 20,000 members of the Ndebele tribe. Mnangagwa has denied responsibility for the mass killings.

"Mnangagwa was the man to execute all of Mugabe's dirty work," said Rugare Gumbo, another liberation fighter who was imprisoned with the two men and worked for the ruling party until a falling out with its leadership in 2014. "We hope when he comes back now, he will be a changed man."

In 2003, Mnangagwa, along with dozens of other members of Mugabe's government, was sanctioned by the United States for "undermining democratic processes or institutions."

He will now be in a position to rebuild his relationship with the West — a critical step in improving the nation's battered economy.

In his short speech on Wednesday, Mnangagwa said, "We need the cooperation of our friends outside the continent" — a bold request for assistance after years of Mugabe's anti-Western rhetoric.

In 2000, a State Department cable described Mnangagwa as "widely feared and despised throughout the country," a man who "could be an even more repressive leader" than Mugabe.

But American officials have indicated that they are prepared to work with Mnangagwa as he assumes the leadership of Zimbabwe.

"We are always willing to give someone a fresh start," the U.S. ambassador to Zimbabawe, Harry K. Thomas Jr., said in an interview. "You can't forget the past. You just cannot. But this is an opportunity for a fresh start. This is an opportunity to show the world that you have the capacity to lead in a free and fair way that's friendly to business as well as the common person."

Thomas described the country as being in an "economic abyss," an assessment that both Zimbabwean and foreign analysts echoed.

At the heart of the country's economic woes was the government's weakening of property rights, which led to the eviction of thousands of white farmers from their lands and the disintegration of the country's once-powerful agricultural sector. Foreign banks fled the country when their loans were not repaid.

Zimbabwe's government then tried to solve the crisis by printing more money. What followed was one of the most extreme cases of hyperinflation in modern times. Eventually, the country was producing 100 trillion-dollar bills in Zimbabwean currency. By 1999, Zimbabwe began defaulting on its foreign debt and lost access to loans from institutions such as the World Bank and International Monetary Fund.

"Mnangagwa needs to give the right to own property back to the people," said John Robertson, a Zimbabwean economist. "If you have an asset base that helps guarantee the repayment of loans, international banks can lend to local banks and local banks can lend to people."

Aside from the economic questions, much else remains unclear in post-Mugabe Zimbabwe.

For now, elections are scheduled in 2018. But Mnangagwa has not said whether he plans to delay them.

Paul Schemm in Addis Ababa, Ethi­o­pia, contributed to this report.