Zimbabwe's opposition leader Morgan Tsvangirai has been sworn in as prime minister in a unity government with President Robert Mugabe.
Mr Tsvangirai won the first round of last year's presidential election - but he withdrew from the run-off, citing violence against his supporters.
However he later agreed to share power with Mr Mugabe.
Problems facing the new government include a cholera epidemic, a collapsed economy and a 90% unemployment rate.
Mr Mugabe administered the oath of office to Mr Tsvangirai at a ceremony in Harare.
Diagram of power-sharing agreement
There were smiles and handshakes as the two bitter political rivals stood face-to-face under a white tent on the grounds of Mr Mugabe's presidential palace.
"I will well and truly serve Zimbabwe in the office of prime minister of the republic of Zimbabwe, so help me God," Mr Tsvangirai said with his right arm raised in a brief ceremony that was broadcast live on television.
Two deputy prime ministers were also sworn in: Thokozani Khupe, the deputy leader of Mr Tsvangirai's Movement for Democratic Change (MDC), and Arthur Mutambara, the leader of a break-away faction of the MDC.
The new prime minister must deal with an economy in ruins and a cholera epidemic which has killed more than 3,400 people.
Hyperinflation is causing prices to double every day and the country stopped publishing inflation figures after it was last estimated at 231m%. People are using foreign currency wherever possible.
More than half the population rely on emergency food aid.
The cabinet in the new coalition government will be sworn in on Friday.
There is deep scepticism about whether it will work, says the BBC's southern Africa correspondent, Peter Biles, in Johannesburg.
At best it will be a transitional arrangement leading eventually to a new constitution and fresh elections, he says.
A final deal on power-sharing was reached in January, after Mr Tsvangirai returned to Zimbabwe following an absence of more than two months for fresh talks with Mr Mugabe.
He had come under heavy pressure from the Southern African Development Community (Sadc), a regional grouping, to agree to a coalition.
Earlier negotiations had faltered after the MDC accused Mr Mugabe's Zanu-PF of keeping the most powerful ministries - including the one that controls the police - to itself.
A minister from each party will now share the home affairs ministry.
Other important ministries the MDC will control are health, education and finance.
On Tuesday, Mr Tsvangirai named Tendai Biti, the secretary general of his Movement for Democratic Change (MDC), as his choice for finance minister.
A Zimbabwean judge last week dropped treason charges against Mr Biti over an alleged coup plot, citing lack of progress in the case against him.
Mr Biti's new position will be a key one, given the country's economic collapse.
Like the rest of the bureaucracy, health, education and finance have no money to spend on reconstruction and the big international donors have said they will not grant substantial additional aid until there are real changes in how Zimbabwe is governed.
"We are ready to support the economic and social recovery of Zimbabwe once the new government shows tangible signs of respect for human rights, the rule of law, and macro-economic stabilisation," the European Union said in a statement released after Mr Tsvangirai was sworn in.