Leaders of Zimbabwe's three main political parties have begun negotiations to implement a stalled power-sharing agreement aimed at ending 10 months of political stalemate. The two major parties appeared to harden their positions on the eve of the talks.

The presidents of South Africa and Mozambique arrived in Zimbabwe and immediately began talks with President Robert Mugabe, Prime Minister-designate Morgan Tsvangirai and the leader of a third party, Arthur Mutambara.

South African President Kgalema Motlanthe, his predecessor Thabo Mbeki and Mozambican President Armando Guebuza are leading a regional effort to resolve differences that have prevented the formation of a unity government mandated by a four-month-old power sharing agreement.

Mutambara told reporters it was time for flexibility, compromise and pragmatism. But he underlined that his group, as an opposition party, supported the demands of Tsvangirai's Movement for Democratic Change (MDC), and he said he hoped Mr. Mugabe's ZANU-PF party would respond positively to these demands.

Mr. Tsvangirai met Sunday with his party leadership, one day after returning home following a two-month absence. MDC Spokesman Nelson Chamisa told reporters the talks were critical, but the party would maintain its demands.

"It is a watershed in terms of bringing to finality and closure the chapter of negotiations," said Chamisa. "And in that regard the MDC is not going to be bulldozed into accepting piecemeal the outcome of those negotiations. We are going to insist on our outstanding issues."

The MDC is demanding a more equitable sharing of key cabinet ministries, the redistribution of provincial governorships and senior government posts, and the release of about 30 activists who have been detained under what it says are trumped-up charges.

Mr. Mugabe also indicated that ZANU-PF would not make further concessions. In an interview with the government-owned Sunday Mail newspaper, the Zimbabwean president said the other parties at the talks would either accept the agreement or break it. He has indicated he was prepared to form a new government on his own if the talks did fail.

Zimbabwe's parliament is due to reconvene Tuesday to debate a constitutional amendment that would legalize September's power sharing agreement.

Under the accord, Mr. Mugabe would remain president while Mr. Tsvangirai would take the newly created post of prime minister. Mutambara would be named one of two deputy prime ministers.

The power-sharing accord was reached after lengthy negotiations that began following disputed elections in which the MDC won a majority of the seats in parliament.

Mr. Tsvangirai received the most votes in the first round of the presidential vote, but Mr. Mugabe won the second round after Mr. Tsvangirai withdrew, citing a campaign of intimidation against his supporters.

The crisis aggravated Zimbabwe's economic problems, characterized by hyper-inflation, high unemployment, a cholera epidemic that has killed more than 2,000 people and food shortages that have humanitarian groups scrambling to provide food aid to more than one half the population.