The people of Zimbabwe are going to the polls in the second round of a presidential election in which Robert Mugabe is the only candidate. The opposition leader, Morgan Tsvangirai, withdrew from the contest because of violence and intimidation against his supporters. He has urged them to vote to protect themselves from further attacks. Mr Mugabe has ignored several calls from other African leaders to postpone the poll. The latest was from the government of Nigeria - Africa's most populous nation - which said it was doubtful that a credible election could be held in the present circumstances. Earlier Mr Mugabe brushed aside calls for the election to be postponed or called off and said his party Zanu-PF would continue to rule the country as they believed it should be ruled. Reports suggested Zanu-PF membership cards were selling for huge sums of money on the black market. Those buying the cards believe they will offer some protection from attack by militias, a BBC correspondent reports. Mr Mugabe came second to Mr Tsvangirai in the first round of the presidential vote in March. Since then, the MDC says some 86 of its supporters have been killed and 200,000 forced from their homes by militias loyal to Zanu-PF. The government blames the MDC for the violence, and on Thursday the police accused Britain and the US of backing MDC plans to disrupt Friday's voting. "There is information that the United Kingdom and the United States of America are planning to use the MDC and civic organisations and others to stage violent demonstrations in order to disrupt the presidential run-off elections due tomorrow," said Senior Assistant Commissioner Faustino Mazango. Mr Mugabe went further, saying he had no quarrel with the British people, but with those in "number 10, Downing Street" - the official address of UK Prime Minister Gordon Brown. "The demons in number 10, Downing Street, must be exorcised by someone," he said. Regional leaders - including from Nigeria, the Southern African Development Community (Sadc) and the African Union - have called on Mr Mugabe to postpone the vote and negotiate with the opposition. While he said he planned to attend an African Union summit in Egypt next week, Mr Mugabe said the AU had "no right in dictating to us what we should do with our constitution, and how we should govern this country". Role of military He told supporters during Thursday's Harare rally that he was not interested in the opinions of the rest of the world. "I would rather the world left us in peace. Sanctions, well, let them continue with their sanctions, but we will find our way of existing," said Mr Mugabe. But he suggested negotiations with the MDC were possible. "Should we emerge victorious, which I believe we will, sure we won't be arrogant, we will be magnanimous and say 'let's sit down and talk,'" he said. Mr Tsvangirai has said negotiations would not be possible if Mr Mugabe went ahead with the run-off. He said Zimbabwe's army was preparing to force people to vote in massive numbers for Mr Mugabe. Speaking to the BBC's World Update programme, Mr Tsvangirai said his supporters should vote if necessary to avoid violent reprisals. "Do not take any risk. It's not worth it. This government is illegitimate. It will remain so." One MDC supporter in Harare told the BBC's Africa Have Your Say programme that he definitely would not vote because the MDC had not been given a fair chance to campaign. A BBC contributor in the southern town of Masvingo said army chief-of-staff Major General Engelbert Rugeje told a rally that his soldiers would force members of the public to go and vote for Mr Mugabe. "We are soldiers who do not ask for things, but force things," Maj Gen Rugeje said. "On Friday, we are going to make sure that you go and vote not for a person of your choice, but Mugabe." Villagers have also described how they have been ordered to feign arm injuries so they can be assisted in polling booths by Zanu-PF supporters, who will ensure they vote for Mr Mugabe.