Mr Mugabe told supporters at a campaign rally in the southern city of Bulawayo on Friday that he would "never allow an event like an election to reverse our independence, our sovereignty."
"Only God who appointed me will remove me - not the MDC, not the British," he said.
The BBC's Peter Biles in Johannesburg says President Mugabe has given another indication that he will not relinquish power as a result of a poll.
Mr Mugabe has accused the MDC of acting in the interest of Britain, the former colonial power, and other Western countries.
He was also quoted by the state-run Herald newspaper as saying that MDC leaders had been compiling names of people they say are victims of political violence.
"They say this so that they can later say the elections were not free and fair. Which is a damn lie!" he said.
Mr Mugabe's police chief, Augustine Chihuri, has claimed that the MDC is the main culprit in the current political violence.
"All necessary force will be applied on malcontents and perpetrators of violence," he said.
"This violence is aimed at intimidating people from voting."
Zanu-PF supporters filmed by US embassy staff in Harare township
The MDC is to announce on Monday whether it will contest the 27 June poll, a party source has told the BBC.
Mr Tsvangirai - who is due to face Mr Mugabe in the run-off - is reported to be under pressure to pull out in view of escalating pre-poll violence.
New footage emerged on Friday, shot by US embassy staff, showing ruling party militias armed with sticks apparently hunting for MDC supporters in a township in the capital, Harare.
Zimbabwe's immediate neighbours have added their voice to increasing international concern over the validity of the run-off.
On Friday Angolan President Jose Eduardo Dos Santos, one of Mr Mugabe's closest allies, has urged him to stop the violence.
Tanzanian Foreign Minister Bernard Membe, head of an election monitoring team, told the BBC earlier this week that violence appeared to be "escalating throughout Zimbabwe".
Nigerian Nobel-winning writer Wole Soyinka told the BBC that Mr Mugabe had ruined Zimbabwe with a "scorched earth policy" and that Zimbabweans were primed to "throw off this yoke by all means necessary".
In Brussels, the European Union has drafted a summit statement saying it is ready to take unspecified "additional measures against those responsible for violence".
A former Zimbabwean police officer tells of how he was threatened
The EU already has an arms embargo against Zimbabwe and has placed travel bans on - and frozen the assets of - President Mugabe and senior government and ruling Zanu-PF party officials.
Mr Mugabe - who has ruled Zimbabwe since independence in 1980 - blames Western sanctions for causing the country's economic freefall.
The MDC suffered at least five violent deaths of activists or their family members this week and its secretary general, Tendai Biti, was charged with treason and subversion.
"Differences of opinion" have emerged among the party's senior officials over its next move, MDC spokesman Nelson Chamisa told the BBC after the leadership met in Harare on Friday.
The party, he said, needed to assess the situation in the country but if conditions did not change, the vote would be a "charade".
"We are assessing the situation as some areas are inaccessible," he added.
"People are being abducted at night. Our grass-roots activists are being subjected to terror. Some of them are staying in the bushes and mountains to avoid pro-government militias.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.