Ethiopian Prime Minister Meles Zenawi has died in hospital abroad, at the age of 57, the government has said.
Leaders from Africa and around the world praised Mr Zenawi, who was a regional strongman in the volatile Horn of Africa.
Mr Meles was battling ill-health for the last year and had not been seen in public since the G20 summit in Mexico in June. He had reportedly been in hospital in Brussels.
Government spokesman Bereket Simon gave no details of the illness.
He said: "He had been recuperating well, but suddenly something happened and he had to be rushed to the intensive care unit and they couldn't keep him alive."
Ethiopian deputy prime minister Hailemariam Desalegn would take over interim power.
Mr Meles was a former rebel fighter who came to power in 1991 after toppling the bloody dictatorship of Mengistu Haile Mariam.
Iron-fisted and austere, Mr Meles was propelled into the club of African rulers in power for more than 20 years by a landslide victory in 2010 elections, where he won 99% of the vote.
From a revolutionary, Mr Meles created a new persona for himself as the champion of Africa's economic and environmental rights on the international scene.
David Cameron described Mr Meles as "an inspirational spokesman for Africa" on global issues.
"His personal contribution to Ethiopia's development, in particular by lifting millions of Ethiopians out of poverty, has set an example for the region," he said.
Former Prime Minister Gordon Brown said that with Mr Meles's death, the world's poor had lost one of their greatest champions.
Tributes also came from African leaders. South African President Jacob Zuma praised Meles as "a strong leader, not only for his country but on the African continent" and the Horn of Africa in particular.
"It is an absolute tragedy for Africa and the people of Ethiopia to mourn such an exceptional leader who contributed as an active role-player in various continental and global initiatives, especially in playing host to the African Union," Zuma said in a statement.
Kenyan President Mwai Kibaki called Mr Meles a "pragmatic and visionary" leader who helped stabilise his country and placed it on the path of economic growth, adding that his death is a "devastating loss".
Mr Meles played a key part in brokering peace efforts between newly-independent South Sudan and its former civil war foe Sudan, a role praised by South Sudan's information minister Barnaba Marial Benjamin.
"He was one person who could say in black and white what the position of both countries was - and was respected by both," Benjamin said.
On paper Mr Meles' government has fostered a policy of ethnic federalism, devolving significant powers to regional, ethnically-based authorities but central control remains firmly in the hands of the ruling party.
His death also leaves a major power gap in the Horn of Africa, with Ethiopia playing a key role in the fortunes of many of its neighbours.
Ethiopian troops invaded Somalia for a second time last year after a US-backed invasion in 2006 and is supporting the fight against al Qaeda-linked Shebab insurgents.
Somali Islamist militants hailed the death of the prime minister as an "historic day" and said Ethiopia, which has troops inside Somalia, would now crumble.
"We are very glad about Meles' death. Ethiopia is sure to collapse," Sheikh Ali Mohamud Rage, the spokesman for Al Shabaab told reporters.
Mr Meles' death could also potentially see changes in the relationship with arch-foe Eritrea, which split from Ethiopian rule in 1993, before spiralling into a bitter 1998-2000 border war.
A peace deal led to a tense standoff, with Mr Meles refusing to pull troops from the border town of Badme, even after an international court ruled the town belonged to Eritrea.
The town has been the source of festering discontent between the two nations ever since.
Ethiopia has declared a state of national mourning but has not fixed a date for a funeral, said government spokesman Bereket Simon, adding that "everything is stable" in Ethiopia following the news.