Harare - Zimbabwean President Robert Mugabe, Movement for Democratic Change (MDC) leader Morgan Tsvangirai and the leader of an MDC splinter faction, Arthur Mutambara, on Monday formally signed an historic agreement to share power in a unity government.

The deal, which comes after eight weeks of negotiations brokered by South African President Thabo Mbeki, ends Mugabe's 28-year monopoly on power.

Under the deal, Mugabe remains president and Tsvangirai occupies a resurrected role of prime minister with the two sharing executive powers in a French-style arrangement. Mutambara becomes deputy prime minister.

Speaking after the signing in a city-centre hotel that was attended by hundreds of politicians from both sides plus journalists and diplomats, Mugabe struck a defiant note, warning the new government would only last as long as Zimbabwe's sovereignty was protected.

Welcoming the MDC to power, he offered to share his government's experiences, both good and bad, with the MDC - a proposal that, given his government's disastrous record over the past decade drew jeers.

Mugabe's contention that the MDC had used "violence" to pursue its objectives also sparked disbelief, given the state's nine-year campaign of violence against the MDC. Since March alone, over 125 MDC supporters have been killed by pro-Mugabe militia and soldiers.

But Mugabe also gave an assurance that "we are committed to this deal. We will do our best."

In a sign of the difficult healing process ahead, around 100 stone- throwing MDC and Zanu-PF supporters clashed outside the hotel after the signing. The melee was broken up by riot police.

The incident took place despite Tsvangirai's impassioned appeal for unity.

"This does not mean we must forget about the past decade of hardship and struggle," he said in a speech, calling for a "public acknowledgement of past wrongs" to kickstart a process of national healing.

Some MDC members see the agreement - which leaves Mugabe key powers, including control of the army - as a weak deal for the party, given that Tsvangirai took the most votes in the last credible presidential election.

Explaining his decision to sign up, Tsvangirai said: "The world has too many examples of when people were driven by past wrongs rather than future glories."

His desire for a new beginning ran "deeper than the scars I bear from the struggle," said the 56-year-old former trade unionist who was badly beaten by police last year for his opposition activities.

To its majority in parliament the MDC adds a slight edge over Zanu- PF in cabinet, with 16 of 31 posts going to the party, including three to Mutambara's faction. Zanu-PF gets 15.

Tsvangirai will chair a new council of ministers that will oversee the formulation and implementation of policy, while Mugabe will continue to chair the cabinet. Details of which party was given which ministry were not immediately released.

For Mugabe, agreeing to share power with the man whom he has long disparaged as a puppet of the West is an unprecedented climbdown.

The agreement is above all a triumph for Mbeki, after many years of fruitless quiet diplomacy with Mugabe. Mugabe lavished praise on the South African, who is in hot water at home over an arms deal, calling his efforts "heroic."

The new government faces the daunting task of rebuilding Zimbabwe's ruined economy. Analysts put the sum needed at between 15 and 25 billion dollars.

Ending devastating food shortages would be his first priority, Tsvangirai said. Aid agencies say 5 million Zimbabweans - nearly half the population - will need food aid by January.

Western governments are adopting a wait-and-see approach before committing to any large-scale rescue plan.

"What is important is how quickly investors move in, otherwise the deal will not change anything at all," said John Robertson, a Harare- based economics consultant. (dpa)