The Sadc gathering is being chaired by South African President Thabo Mbeki, who has been mediating in the talks.

Both Zimbabwean President Robert Mugabe and opposition leader Morgan Tsvangirai are in Johannesburg for the summit.

But Botswana's President Seretse Khama Ian Khama has refused to attend because of Mr Mugabe's presence.

Botswana's government said he did not want the Southern African Development Community (Sadc) to give legitimacy to the widely condemned second round of Zimbabwe's presidential election in June, in which Mr Mugabe was the only candidate.

South Africa's trade unions, Cosatu, are planning to lead a march in protest at Mr Mugabe's participation in the Johannesburg meeting.

President Mbeki is expected to brief his fellow heads of state on the power-sharing negotiations in Zimbabwe at the summit.

Before the meeting, the South African leader held talks separately with Mr Mugabe, Mr Tsvangirai, leader of the Movement for Democratic Change (MDC), and Arthur Mutambara, head of a breakaway MDC faction.

A spokesman for the Zimbabwean President, Patrick Chinamasa, has said negotiations will resume on the margins of the summit.

Mr Chinamasa told Zimbabwe's state-run Herald newspaper that there was increasing pressure within the country to convene parliament and form a new government.

"We cannot continue wandering around without direction," he said.

But an aide to Mr Tsvangirai insisted prospects for any further talks at the summit depended "on the sincerThe MDC leader arrived in Johannesburg on Friday after being temporarily stopped from flying when his passport was briefly confiscated at Harare airport. The Zimbabwe government refused to comment on the incident.

Mr Mbeki said the power-sharing talks in Harare were adjourned on Tuesday to allow Mr Tsvangirai time for reflection.

Mr Mbeki also denied that there was a breakdown in negotiations after reports that Mr Mutambara had signed a separate deal with Mr Mugabe.

Mr Tsvangirai warned that "a resolution that represents anything other than the will of the Zimbabwean people would be a disaster for our country".

The MDC leader finished ahead of Mr Mugabe in the first round of the presidential election in March and the party also won a majority in parliamentary elections.

But Mr Tsvangirai pulled out of the second round of the presidential election, citing a campaign of violence against his supporters.

Sticking points in the power-sharing talks are reported to include:

• the balance of power between Mr Mugabe and Mr Tsvangirai

• the makeup of any coalition cabinet

• control of Zimbabwe's security forces

• the possibility of an amnesty over post-election violence

The BBC's Jonah Fisher in Johannesburg says Zimbabwe's problems have spilled far beyond its borders, so fears that Sadc leaders have about its economic freefall and political deadlock are much more than just neighbourly concern.

Mr Mbeki pushed government and opposition leaders for three long days this week, but a power sharing deal remains elusive and his approach will once again be under scrutiny, our correspondent says.

There are many who think Mr Mbeki, who continues to pursue what he calls a policy of "quiet diplomacy", just has not pushed his Zimbabwean counterpart hard enough, he adds.

Mr Mbeki has said he is determined to reach a deal and has promised to stay in Zimbabwe for six months to get one if necessary.

Correspondents say Botswana's unprecedented move shows mounting opposition to Mr Mugabe's rule among Zimbabwe's neighbours.

Its government said Mr Mugabe should not be allowed to attend Sadc gatherings until his Zanu-PF party reached a settlement with opposition groups.

"We agree with the mediation, but we don't think that people who are in these negotiations... should be attending as if they have won an election," Botswana Foreign Minister Phandu Skelemani told the BBC.

Sadc Executive Secretary Tomaz Salomao said he respected Botswana's decision, but insisted the remaining 13 members could still make binding decisions.

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