By Nelson Banya

HARARE (Reuters) - Zimbabwean opposition leader Morgan Tsvangirai said on Thursday there could be no negotiations with President Robert Mugabe if he went ahead with a one-man election on Friday.

Tsvangirai said if Mugabe declared himself president and extended his 28 years of uninterrupted rule he would be shunned as an illegitimate leader who killed his own people.

Africa's most iconic figure, Nelson Mandela, added his voice to a storm of African and international condemnation of the violence and chaos in Zimbabwe, in a rare political statement that showed the level of concern around the continent.

Mugabe and his officials have remained defiant, however, saying the vote is a legal obligation.

Deputy Information Minister Bright Matonga told al Jazeera television: "People are going to vote tomorrow. There is no going back." He said Tsvangirai should be out campaigning instead of trying to set conditions for Mugabe.

Zimbabwean police said Britain and the United States were backing plans by Tsvangirai's Movement for Democratic Change (MDC) and some NGOs to disrupt Friday's vote with violence, including burning down voting tents.

Assistant Commissioner Faustino Mazango told a news conference: "It is evident that the opposition MDC has plans to disrupt the election. These counter-productive criminal activities will be met head-on and with the full force of the law."

He said the plot was revealed by five people arrested on Wednesday.

Tsvangirai, who withdrew from Friday's run-off last Sunday and took refuge in the Dutch embassy, tried to step up the pressure by telling Mugabe that his chances of negotiating an end to Zimbabwe's catastrophic collapse would end on Friday.

"Negotiations will be over if Mr Mugabe declares himself the winner and considers himself the president. How can we negotiate?" Tsvangirai told London's Times newspaper.

MBEKI CHALLENGED

In a later interview with Sky News, Tsvangirai challenged South African President Thabo Mbeki, the designated regional mediator in Zimbabwe, to take urgent action to end the crisis.

Mbeki, leader of Africa's biggest economic power, has been widely criticised for being soft on Mugabe despite a crisis that has flooded his country with millions of refugees.

"I hope that given ... the degeneration of the crisis we are facing, he will also act in terms of it being urgent to fulfil his mandate," Tsvangirai said.

Tsvangirai's lieutenant Tendai Biti was released on bail on Thursday after being held for two weeks on treason charges. Bail was set at 1 trillion Zimbabwean dollars -- about $90, his lawyer said.

Mugabe, president since independence from Britain, has presided over Zimbabwe's slide from one of the region's most prosperous nations to a basket case with inflation estimated to have hit at least 2 million percent.

He blames the crisis on sanctions by Britain and other Western countries.

Mugabe is facing a concerted international campaign to push him into calling off the vote by threatening he will be shunned by the world, including African allies once over-awed by his liberation hero status.

Mandela said in a speech at a dinner for his 90th birthday in London that there had been a "tragic failure of leadership in our neighbouring Zimbabwe."

Tsvangirai told Mugabe that if he came to him after the vote he would tell the veteran leader: "I made these offers, I made these overtures, I told you I would negotiate before the elections and not after -- because it's not about elections, it's about transition.

"You disregarded that, you undertook violence against my supporters, you killed and maimed ...How can you call yourself an elected president? You are illegitimate and I will not speak to an illegitimate president."

On Wednesday, a security committee of the Southern African Development Community (SADC) -- urged postponement of the vote, saying Mugabe's re-election could lack legitimacy in the current violent climate.

But Zimbabwe's state media on Thursday quoted SADC poll monitors -- the only large group in the country -- as saying they would stay for the vote despite Tsvangirai's withdrawal.

South Africa said a top negotiator was in Harare mediating talks on options including calling off the election.

The ruling African National Congress, which has been severely critical of Mugabe, in contrast to Mbeki, said it was not too late to call off the vote.

"The ANC is convinced that it is not too late for President Mugabe to cancel the election, the run-off, and lead the country in a dialogue that will be for the good of all Zimbabweans," spokeswoman Jesse Duarte told BBC television.