* Mugabe rejects African calls to stop election

* Tsvangirai says no negotiations if vote goes ahead

* People will be forced to vote, says Tsvangirai

* Nigeria adds its weight to call for postponement

By Cris Chinaka

HARARE, June 26 (Reuters) - Zimbabwean President Robert Mugabe on Thursday rejected African calls to postpone a presidential election on Friday, saying there could be no interference in his country even from the African Union.

Mugabe, 84, who is bound to extend his 28-year-rule in the one candidate election, said he was open to discussions with the opposition MDC. Its leader Morgan Tsvangirai has withdrawn from the election after a wave of deadly attacks on his supporters.

Addressing a campaign rally in Chitungwiza, south of Harare, Mugabe said: "We have some of our brothers in Africa making that call (to postpone the vote), pushing us to violate our own law and we have refused to do so, we are sticking to our law."

Mugabe said he would attend an African Union summit in Egypt next week but no solutions could be imposed on Zimbabwe from outside. He said he was ready to answer any challenge from within the AU to the elections.

"I know some people are gearing themselves for an attack on Zimbabwe. I want to see any country which will raise its finger in the AU, our elections have been free."

A security committee of the Southern African Development Community (SADC) on Wednesday called for the vote to be postponed, saying Mugabe's re-election as the only candidate could lack legitimacy because of chronic political violence.

The committee includes AU chairman Tanzania.

Nigeria, an African heavyweight, backed the SADC's call, saying it was doubtful a credible poll could be held under the current circumstances.

"Nigeria joins the Southern African Development Community and well meaning friends of Zimbabwe in calling for postponement of the run-off elections," the Foreign Ministry said late on Thursday.

Opposition leader Tsvangirai last Sunday pulled out of the vote because of violence that he says has killed almost 90 of his Movement of Democratic Change supporters. He has taken refuge in the Dutch embassy ever since.

Tsvangirai said earlier there could be no negotiations with Mugabe if he went ahead with Friday's election. He said that if Mugabe declared himself president he would be shunned as an illegitimate leader who killed his own people.

But he said people would be forced to vote in the election. "What will happen tomorrow is that people will be forced to vote ... because the military were mobilised to accompany this process," Tsvangirai said in an interview with Portuguese radio station Renascenca, which gave Reuters a transcript.

MANDELA JOINS CONDEMNATION

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.

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

U.N. Secretary-General Ban Ki-moon reiterated that violence and intimidation must be ended and that the election should be postponed.

"The situation seems to be evolving, but I am still concerned that if the election is held under these circumstances, there will be serious questions and controversies about the legitimacy," he told reporters in New York.

Diplomats at the U.N. in New York said the Security Council might call an emergency meeting over the weekend if the election goes ahead and Mugabe declares himself the winner.

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.

A loaf of bread now costs 6 billion Zimbabwe dollars, or 150 times more than at the time of the first round of elections on March 29 when Tsvangirai defeated Mugabe but fell short of the absolute majority needed for an outright victory.

Mugabe blames the crisis on sanctions by former colonial ruler Britain and other Western countries.

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

Mugabe said he was open to discussions with other parties but stressed that any solution to the crisis in the country needed to come from Zimbabweans.

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.

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. (Additional reporting by MacDonald Dzirutwe and Nelson Banya in Harare, Louis Charbonneau at the United Nations, Susan Cornwell in Kyoto; Writing by Barry Moody and Marius Bosch; Editing by Richard Balmforth)

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