Archbishop Desmond Tutu has urged the international community to intervene in Zimbabwe - by force if necessary.
The former Cape Town archbishop said he would support the deployment of a UN force to restore peace in the country.
He said African Union leaders should refuse to recognise Robert Mugabe as the legitimate president of Zimbabwe.
Meanwhile, President Mugabe declared he was heading for a "sweeping victory", in remarks made at a relative's funeral late on Saturday.
"The returns show that we are winning convincingly, that we have won in all the 26 constituencies in Harare, an MDC stronghold where we won in only one constituency in March. That is the trend," he said in footage broadcast on state television.
It is thought Mr Mugabe will be sworn in for another term on Sunday, although final results from the one-candidate election have been delayed.
The opposition boycotted the vote amid claims of violence and intimidation.
Mr Mugabe was said to have won by a wide margin, but international observers have reported many spoilt ballots, which in some areas could outnumber votes cast.
Earlier, officials said the count was complete, but later reports said results from rural areas were still trickling in.
In an interview for the BBC's Andrew Marr programme, Archbishop Tutu said the African Union could have a clear role in persuading Mr Mugabe to negotiate.
"If you were to have a unanimous voice, saying quite clearly to Mr Mugabe... you are illegitimate and we will not recognise your administration in any shape or form - I think that would be a very, very powerful signal and would really strengthen the hand of the international community."
"I think that a very good argument can be made for having an international force to restore peace," he added.
African Union (AU) foreign ministers have gathered in the Egyptian town of Sharm el-Sheikh, before a full meeting of heads of state on Monday which Mr Mugabe is expected to attend.
Earlier the Kenyan Foreign Minister Moses Wetangula said sanctions against Zimbabwe are unlikely to work, and that Mr Mugabe and the opposition should instead be encouraged to talk.
Push for negotiations
Morgan Tsvangirai, leader of the Movement for Democratic Change (MDC), announced he was pulling out of the election on Sunday.
But his name remained on ballot papers after Zimbabwe's electoral authorities refused to accept his decision.
In interviews published in British newspapers on Sunday, Mr Tsvangirai said he would push for negotiations with Mr Mugabe on a new constitution and fresh elections.
"We have the power to control parliament, and that is recognised even by Mugabe's Zanu-PF... We must force a transitional agreement for a set time-frame and work towards a new constitution for Zimbabwe," he told the Mail on Sunday.
"I am confident we can achieve that if international pressure keeps up," he added.
In a separate interview with the Sunday Telegraph, Mr Tsvangirai said it was possible that Mr Mugabe could remain as a ceremonial head of state.
"I don't think it's inconceivable for such an arrangement to include him, depending, of course, on the details of what is being proposed and what are the arrangements," he said.
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.
Are you in Zimbabwe? What did you think of the run-off? What do you think should happen now? Tell the BBC your experiences by using the form below. Let us know if you would prefer us not to use your real name.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.
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.