Ethiopian Prime Minister Meles Zenawi has died aged 57 in a hospital abroad, the government says.
It did not give details but an EU spokesman later told journalists Mr Meles had died in Brussels, Belgium.
Mr Meles had not been seen in public for weeks and speculation about his health mounted when he missed a summit in Addis Ababa last month.
His deputy, Hailemariam Desalegn, becomes prime minister until elections due in 2015, state media reported.
Mr Meles took power as the leader of rebels that ousted communist leader Mengistu Haile Mariam in 1991.
He is credited with spearheading economic development in Ethiopia - but, critics charge, at the price of respect for human rights.
He was austere and hardworking, with a discipline forged from years spent in the guerrilla movement - and almost never smiled, says Elizabeth Blunt, the BBC's former correspondent in Addis Ababa.
Liberian President Ellen Johnson Sirleaf said Mr Meles was an "intellectual leader for the continent", and UK Prime Minister David Cameron called him "an inspirational spokesman for Africa" who had lifted millions out of poverty.
But a spokesman for al-Shabab Islamist militants in Somalia - where Mr Meles twice sent troops to fight - told Reuters news agency they were "very glad" of his death, saying Ethiopia was "sure to collapse".
Ethiopia's Council of Ministers announced "with great sadness the untimely death of Prime Minister Meles Zenawi" in a statement, state TV reported.
The statement said Mr Meles had been receiving medical treatment abroad for the past two months and that his health was improving. But he developed a "sudden infection" on Sunday and despite emergency treatment, died at 23:40 on Monday.
European Commission spokesman Olivier Bailly later told a regular news briefing that Mr Meles "passed away during the night here in Brussels".
Over some eight weeks that Mr Meles had been absent from the public eye, concerns about his health had grown, and in July he was said to have been admitted to hospital.
Reports suggested he was in hospital in the Belgian capital suffering a stomach complaint but these were never confirmed by the Ethiopian authorities.
Three weeks ago, government spokesman Bereket Simon dismissed reports Mr Meles was critically ill, and declined to give any details about Mr Meles's whereabouts.
A period of mourning had been declared until the funeral, for which no date has been announced.
State television said his body would be flown to Addis Ababa later on Tuesday.
Mr Hailemariam, who is also Ethiopia's foreign minister, will become acting prime minister, government spokesman Bereket Simon told reporters.
He said an election was not necessary as the "constructional procedure" allowed for Mr Hailemariam to "kick off as a full-fledged prime minister".
In an earlier news conference, Mr Bereket said Mr Meles had struggled with illness for a year, but he had continued to work regardless.
Concerns have been expressed - including by Kenyan Prime Minister Raila Odinga - that Mr Meles's death could lead to a power vacuum and dangerous instability in Ethiopia.
But Mr Bereket insisted the country was stable and that "everything will continue as charted" by the late prime minister.
This theme was echoed by state television, which stated that "even if Ethiopia has been badly affected for missing its great leader, Prime Minister Meles Zenawi initiated fundamental policies and strategies which will be further strengthened".
Ethiopia's economy has grown rapidly in recent years, despite the secession of Eritrea and the subsequent war between the two countries.
Under Mr Meles, Ethiopia became a staunch US ally, receiving hundreds of millions of dollars in aid over the years, and hosting the US military drones that patrol East Africa.
He won accolades from the West for sending troops to battle Islamist militants in Somalia, says the BBC's James Copnall.
But concern had been growing about the lack of democracy and human rights in Ethiopia, our correspondent in the region says.
At least 200 people died in the violence that followed the 2005 elections, and many journalists and politicians have been locked up.
One rights critic, Commander Assefa Seifu, called Mr Meles "a devil incarnate".
"He was always talking about democracy, civil rights, adherence to [the] constitution and the like. But it was only a lip service," he told the BBC.