(Adds White House comments)

By MacDonald Dzirutwe

HARARE, July 5 (Reuters) - South African President Thabo Mbeki met Zimbabwean President Robert Mugabe on Saturday to try to help end a political crisis after a violent election that extended Mugabe's 28-year rule.

The main opposition party, the Movement for Democratic Change (MDC), said its leader Morgan Tsvangirai had declined to meet Mbeki, who has tried to mediate between the two sides.

Tsvangirai and his MDC have criticised Mbeki's mediation efforts, accusing him of siding with Mugabe and have asked the African Union to send an envoy to help with the talks.

Mugabe, in power since 1980, says he supports Mbeki's role in the mediation but has remained defiant in the face of growing condemnation from Western governments and even African neighbours after his disputed re-election on June 27.

"It is the view of the facilitators and the Zimbabwean leadership that we need to move with speed," Mbeki told reporters after a brief meeting with Mugabe and Arthur Mutambara, who leads a breakaway faction of the MDC.

"We agreed that MDC Tsvangirai has to be part of the negotiations, so we are hoping that the process will take place with them."

Mugabe said on Friday the MDC must drop its claim to power and accept he was the rightful head of state. He said Zimbabwe's crisis, which has ruined the economy and sent millions of refugees into neighbouring states, must be settled internally.

A spokesman for Tsvangirai's MDC, Nelson Chamisa, said the party was "mandated to negotiate under the resolutions of the Africa Union and the Southern Africa Development Community ... on the basis that there is accountability (and) transparency."

"If we were meeting Mugabe as head of (the ruling party) ZANU-PF no problem but not as head of state because we would have endorsed him but you know that his position is in dispute," Chimasa said.

VOTE RIGGING

Mbeki's trip follows a June 27 runoff election, in which Mugabe was the only candidate after Tsvangirai pulled out citing state-sponsored violence against MDC candidates and supporters.

"We will of course engage the AU (African Union) and I am quite certain that they will make their own contribution to move the process forward," said Mbeki.

A White House official said on Saturday that Zimbabwe was certain to come up at the Group of Eight summit in Japan on July 7-9, which will also be attended by the heads of seven African states.

"I think the G8 will strongly condemn what Mugabe has done," Dennis Wilder, a senior National Security Council official, told reporters aboard Air Force One as President George W. Bush was on his way to Japan.

"It will strongly condemn the legitimacy of his government and his governing of Zimbabwe."

Justice Minister Patrick Chinamasa, who leads the ZANU-PF negotiating team, criticised Tsvangirai for failing to attend Saturday's meeting, accusing him of behaving like a rebel.

"I think that what is becoming clear is that if the country is not careful it will be precipitated into a period of instability," Chinamasa told state television.

A film secretly taken by a Zimbabwe prison guard and smuggled out of the country shows rigging that took place for the June 27 run-off vote, Britain's Guardian newspaper said on Saturday.

The film taken by Shepherd Yuda using a camera supplied by the Guardian showed prison staff being told by a war veteran how to fill in their ballot papers for Mugabe. (Editing by Phumza Macanda, Ibon Villelabeitia and John O'Callaghan)

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