Zimbabwe was plunged into fresh political crisis as Prime Minister Morgan Tsvangirai vowed to fight a unilateral decision by President Robert Mugabe to hold elections on July 31.
Mugabe used temporary presidential powers to set the date for a vote that would put an end to their uneasy power-sharing government.
"Today, early in the morning, I received a letter from president Mugabe (proclaiming)... July 31 as the election day," Tsvangirai said.
"President Mugabe is acting unlawfully and unconstitutionally," said his long-time political rival. "As prime minister I cannot and will not accept this."
Tsvangirai vowed to take the matter to court and urged southern African regional leaders to step in and stop Mugabe, 89, who has led Zimbabwe since independence in 1980.
While Mugabe's move would comply with a Constitutional Court order to hold elections by the end of July, the date of the vote is fiercely contested.
Tsvangirai, 61, has vowed to veto any election date that comes before democratic reforms are put into place, fearing Mugabe's ZANU-PF party may once again attempt to manipulate the vote.
In 2008 Tsvangirai led Mugabe in the first round of the presidential ballot, but withdrew from the run-off amid violence that left scores of supporters dead and thousands injured.
Tsvangirai called on regional leaders of the Southern African Development Community (SADC) -- which pushed the pair into a unity government after the violence of 2008 -- to act when they meet in Mozambique on Saturday.
"SADC has the responsibility of ensuring that they call the president to order," he said.
He described Mugabe's move as "an unmitigated frontal and rear attack" on the 15-member regional grouping.
The International Monetary Fund meanwhile announced it would work with Zimbabwe for the first time in more than a decade, a key step in normalising relations with a country whose fragile economy is threatened by massive public debt as well as political strains.
The IMF said it is not planning to provide Zimbabwe -- which had been slapped with restrictions for falling behind on repayments -- with any new funds. But Managing Director Christine Lagarde has approved monitoring of the country's economic programme this year, it said in a statement.
The IMF stripped Zimbabwe of its voting rights in 2003 and nearly expelled the country, a rare move for the Washington-based institution, in 2006.
Though the power-sharing government has made considerable progress in stabilising the economy since an outbreak of blistering hyperinflation ended in 2009, a strong double-digit rebound in growth appears to have ended, slowing to about 4.5 percent in 2012, the IMF said.
Under the power-sharing deal, Mugabe must consult Tsvangirai's Movement for Democratic Change (MDC) about the date of the presidential, parliamentary and local elections.
But Trevor Maisiri of the International Crisis Group said the rival parties have different interpretations of what that means.
"If you speak to ZANU-PF they'll say consultation means the president can ask the principals what they think. With their suggestions he can make a decision," Maisiri said.
"The MDC say 'we'll make the decision together'."
Some commentators argue the MDC has reason to fear an early vote.
After spending much of the last four years focused on government rather than party politics, polls show its star has faded among many voters.
In his tersely worded letter to Tsvangirai, Mugabe justified the use of extraordinary powers to bypass normal decision-making.
Given the Constitutional Court deadline, he said, "it became inexpedient to await the passage through parliament of the electoral amendment bill."
While many reforms have been carried out by the power-sharing government, including the passing of a new constitution, Tsvangirai argues that electoral, media and security laws still need to be implemented to ensure a free vote.
The reforms include ensuring that the electoral roll is updated with eligible voters as well as ensuring that the media can report independently without the fear of being charged for defaming the government.