By Peter Wonacott

JOHANNESBURG-Meles Zenawi, Ethiopia's autocratic leader who guided his poor nation closer to prosperity through a steady economic opening, died late Monday, presenting a tricky leadership transition for a crucial U.S. ally in Africa.

Mr. Meles, who was 57 years old and had served in office, as president and then as prime minister, for 21 years, will be succeeded by Deputy Prime Minister Hailemariam Desalegn, Ethiopia's deputy prime minister, the government said Tuesday.

Although Mr. Meles appointed Mr. Hailemariam his political heir, the new leader has no popular support base of his own and will be surrounded by seasoned operatives who were loyal to Mr. Meles. His level of backing among the military and intelligence communities is unclear. Elections for a new prime minister aren't expected until 2015, said Bereket Simon, Ethiopia's communications minister. The country's recent history of violence-marred votes raises the prospect of a rocky political period ahead.

Mr. Meles succumbed to an infection stemming from an illness that the Ethiopian government wouldn't disclose. Mr. Bereket wouldn't say where he had died, but a European Union official said that Mr. Meles had been in and out of a hospital in Brussels for weeks. An Ethiopian flag-covered coffin with his body arrived in the capital Addis Ababa on Tuesday, news reports said.

The question for Ethiopia now is whether the country can continue on the path set by the late prime minister, whose economic success had been widely admired in Africa even as his government was feared at home.

"He's managed to do what no other African strongman has done," said Jakkie Cilliers, executive director for the Institute for Security Studies, a think tank in Pretoria. "He was the living embodiment of the developmental state."

The succession stakes are high for Washington. Mr. Meles proved an effective yet prickly ally of the U.S., one of Ethiopia's largest donors. During his time in power, Mr. Meles sought an end to conflicts in Sudan and Somalia, and remained a staunch backer of counterterrorism efforts in East Africa.

Even his critics credit Mr. Meles with keeping his country together through famine and war. Ethiopia fought Eritrea in the late 1990s and the conflict continues to simmer. Ethiopia now is seen as an anchor of an unstable East Africa region. The prime minister's death isn't expected to change that.

Mr. Meles, a former rebel leader, combined iron-fisted political controls with the loosening up of lucrative industries. Those moves attracted investment in agriculture and manufacturing, helping to ease chronic food shortages and to transform Ethiopia into one of the continent's fastest-growing economies.

Yet human-rights activists criticized the prime minister for crushing dissent-jailing journalists, blocking critical websites and marginalizing the political opposition.

Mr. Meles served as president in 1991, and became prime minister in 1995. He won again in 2000 and 2005, but those latter elections were marred with violence. When opposition supporters objected to the 2005 vote as not free and fair, police opened fire in Addis Ababa, killing 193 and wounding hundreds. Thousands of opposition leaders and supporters were rounded up and detained. In 2010, Mr. Meles was re-elected and swept all but two of the 546 declared seats in Parliament in a vote that again was criticized as flawed by the opposition.

In his fight against famine, the prime minister threw his support behind Ethiopia's first commodities exchange so farmers across the country could move goods to meet rising demand and curb predatory pricing. The exchange's chief executive, Eleni Gabre-Madhin, eschewed a fully electronic exchange system for a trading floor, so that even the illiterate and those without access to the Internet could sell their goods. "This wasn't a prestige project-he wanted the exchange to benefit small farmers," she said of the late prime minister. "I think we forged a different model in Africa."

Mr. Meles's brand of economic liberalization laid a new foundation for private business and economic growth. The number of private-sector companies in Ethiopia climbed to more 45,000 from almost zero in 1991 when communist rule ended.

Although it remains among the world's poorest countries, Ethiopia's growth exceeded 8% from 2000 to 2010, according to the World Bank.

Mr. Meles also has proved an effective yet prickly ally of the U.S., one of Ethiopia's largest donors. During his time in power, Mr. Meles sought an end to conflicts in Sudan and Somalia, and remained a staunch backer of counterterrorism efforts in the East Africa region.

Even his critics credit Mr. Meles with keeping his country together through famine and war. Ethiopia fought Eritrea in the late 1990s and the conflict continues to simmer. Ethiopia now is seen as an anchor of a shaky East Africa region. Mr. Meles's death isn't expected to change that.

The U.S. provided Ethiopia more than $1 billion in 2010-and $880 million last year-mostly to help improve the country's health situation and food security. The lavish assistance, however, didn't deter Mr. Meles from lashing out at Washington's criticism of his human-rights record, or from tweaking the U.S. by contrasting its conditional aid with China's no-strings-attached financial assistance. He frequently expressed admiration for how Beijing navigated the economic opening of a poor, agrarian country-much like his own.

"These kind of situations in many African counties often result in instability and chaos, but my gut reaction is that's not going to be the case," said David Shinn, the former U.S. ambassador to Ethiopia. "The institutions that Meles has put in place over the past 20 years are surprisingly strong."

Mr. Meles was born on May 8, 1955, in northern Ethiopia. He studied at Addis Ababa University for two years, but cut short his studies in 1974 to join the Tigray People's Liberation Front, a political party.

In the following years, he rose through the ranks of the party and became chairman of the TPLF and the Ethiopian People's Revolutionary Front, an alliance of political movements in Ethiopia. He became president in 1991 after the defeat of the military junta.

In Addis Ababa on Tuesday, Ethiopians were still coming to grips with Mr. Meles's death and what it would mean politically. Many were seen listening to radios and watching televisions for the latest news bulletins. Until recently, the government assured the public the prime minister would return to work soon.

"When I heard first I did not believe it," said Habtamu Tekalegn, 24 years old, a disc jockey at a local bar in Addis Ababa. "At this time, there is no one who has a capacity to contribute to a country like him."

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