(Adds union march, paragraph 9, details)

By Nelson Banya

HARARE, Aug 14 (Reuters) - Botswana's President Seretse Khama Ian Khama will not attend a regional summit if Zimbabwe's ruling party and opposition fail to reach a power-sharing agreement, Botswanan officials said on Thursday.

The stand underlines growing pressure from southern African leaders on Zimbabwean President Robert Mugabe and the opposition to secure an agreement aimed at ending Zimbabwe's post-election political turmoil.

"If there is no agreement on the government in Zimbabwe, the president will not attend the summit," Chris Maribe, director of information at Botswana's foreign ministry, told Reuters. The Southern African Development Community (SADC) summit is due to take place at the weekend in Johannesburg.

Zimbabwean authorities briefly confiscated opposition leader Morgan Tsvangirai's passport on Thursday, threatening to prevent him attending the summit in neighbouring South Africa, an MDC official said. Zimbabwe is likely to be high on the agenda.

Tsvangirai's departure from Harare for the summit was delayed when Central Intelligence Organisation agents confiscated the emergency travel documents he was using, along with the passports of other MDC officials in his party.

MEDIATION STRUGGLE

The MDC leader has been using emergency travel documents since the authorities refused in June to renew his passport after it expired.

Tsvangirai and his team left for South Africa on Thursday evening after his documents were returned, MDC spokesman Tapiwa Mashakada said.

The incident is likely to embarrass South African President Thabo Mbeki, who has been mediating in the Zimbabwe talks. He has dismissed criticism that he is too soft on the defiant Mugabe, saying a tougher approach will only aggravate problems.

The South African leader will face fresh domestic pressure on Saturday when the powerful COSATU trade union federation leads a march to the convention centre where the summit will be held to protest against Mugabe's expected presence.

MDC Secretary-General Tendai Biti said the airport incident should send a strong message to SADC leaders.

"We have been trying to tell President Mbeki about things like this, and people wouldn't believe us. But now here it is ... for all to see."

The political stalemate has worsened an already dire economic crisis. Zimbabwe has the world's highest inflation rate, 80 percent unemployment and shortages of basic goods.

Tsvangirai told reporters at the airport before the incident he was sure talks with Mugabe's government would resume.

Asked if he was still optimistic about a deal, Tsvangirai said: "Oh, yes, of course, we got our independence after how many talks? Hundreds and tens of meetings had been held."

The talks on power-sharing began last month after Mugabe's unopposed re-election in a vote in June that was condemned around the world and boycotted by Tsvangirai because of attacks on his supporters. But three days of meetings in Harare failed to reach an overall deal.

Tsvangirai has said Zimbabwe's post-election government should be based on the March 29 first-round presidential election -- which he won, but not by a clear majority. Mugabe says the MDC should accept the result of the June 27 run-off.

"I am there (at the talks) to protect the will of the people and we are taking a principled stand. I maintain a principled stand to defend the will of the people," Tsvangirai said on Thursday.

Arthur Mutambara, whose break-away MDC faction has 10 seats in parliament, has agreed to power-sharing with Mugabe, but a deal that excludes Tsvangirai would be unlikely to end the crisis. (Writing by Marius Bosch and MacDonald Dzirutwe; Editing by Michael Georgy and Tim Pearce)

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