Zimbabwe is currently gripped by economic collapse and a cholera epidemic. The UN on Thursday reported that the death toll from the disease had risen to 1,123 and that 20,896 people had been infected.
UK Prime Minister Gordon Brown said on Friday the antiquated methods being used to treat the epidemic could not bring hope to the suffering of Zimbabweans, and called for more aid to be sent.
"I believe the situation, contrary to what President Mugabe says, from all the evidence we have is deteriorating and deteriorating rapidly," he told a news conference in London.
US Assistant Secretary of State for African Affairs Jendayi Frazer meanwhile said there was "a complete collapse right now" in Zimbabwe, and said Mr Mugabe needed to step down.
'Pack of lies'
But in a defiant speech at Zanu-PF's annual conference in Bindura, the president insisted "the only persons with the power to remove Robert Gabriel Mugabe are the people of Zimbabwe".
"I will never, never, never surrender. Zimbabwe is mine, I am a Zimbabwean. Zimbabwe for Zimbabweans," he said.
WHERE AFRICA STANDS
Critical of Robert Mugabe - Botswana's president has called for fresh elections - Kenya's PM wants African governments to oust Mr Mugabe - Senegal's president says Mr Mugabe should give up power - Zambia's late president called the region's silence over election violence "scandalous" - Nigeria's foreign minister says he told Mr Mugabe to go in June
Pro power-sharing - Southern African Development Community (Sadc) maintains power-sharing is the only solution - South Africa, the regional powerhouse, backs Sadc - The African Union says a unity cabinet is the only way forward
Mr Mugabe said international criticism of his government's handling of the cholera outbreak was "a pack of lies".
"I won't be intimidated. Even if I am threatened with beheading, I believe this and nothing will ever move me from it: Zimbabwe belongs to us, not the British," he added.
He also questioned whether any of his country's neighbours would "have the courage to order a military intervention".
"What would they come and do militarily here? All that they would come and really pose is a threat to our stability," he said.
"There would be an unnecessary war started in a foolish manner because of foolish persuasion coming from foolish sources."
Senegalese President Abdoulaye Wade became the latest in an increasing number of senior African politicians calling for Mr Mugabe to quit earlier on Friday.
He told the French newspaper, La Croix, that he had supported Mr Mugabe in the past but was forming the view that the president was now the cause of his country's problems.
Also in his speech on Friday, the Zimbabwean leader said he had written to Mr Tsvangirai, inviting him to become prime minister as part of the inclusive power-sharing government, but expressed doubt whether he would accept.
"I have sent letters so that they can come and I can swear [in] and appoint them. We have not reached a stage where we can say with a degree of certainty that they want to be part of this," he said.
Mr Tsvangirai has threatened to withdraw from the power-sharing talks
MDC officials told the Reuters news agency that they had received no such letters.
The two rivals signed a power-sharing deal in September, under which Mr Tsvangirai would have become prime minister and head a new council of ministers, but they have been unable to agree on the distribution of key ministries.
Earlier, Mr Tsvangirai said Mr Mugabe had repeatedly broken the spirit of the agreement.
He said the president was trying to stay in power at all costs, and threatened to suspend all contact with the Zanu-PF unless there was an end to the abduction of MDC supporters and civil society activists.
"More than 42 members have been abducted," the MDC leader told a news conference in Botswana, where he is currently based.
"If these abductions do not cease immediately and if all abductees are not released or charged in a court of law by 1 January 2009, I will be asking the MDC's national council to pass a resolution to suspend all negotiations and contact with Zanu-PF."
Mr Tsvangirai said there could be no meaningful talks while a campaign of terror was being waged to undermine the MDC's support and reduce it to a junior partner in the new government.
BBC Southern Africa correspondent Peter Biles says that this represents a significant shift in Mr Tsvangirai's position, as he had previously remained committed to the power-sharing talks despite a number of reservations.
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