Mugabe accepted the results and quickly confirmed that he would take part in the run-off.
"The president accepts the result as announced and is offering himself for election in the [im]pending presidential run-off," Emmerson Mnangagwa, who has been acting as Mugabe's election agent, said.
But he also cast doubt on the validity of the results, which had put Tsvangirai ahead.
"The presidential result as announced do not reflect the genuine expression of the will of the Zimbabwean people given the many anomalies, malpractices, deflation of figures relating to Zanu-PF candidates," he said.
If Tsvangirai refuses to take part in a second round, Mugabe would keep his hold on power according to electoral law.
Kalay Maistry, Al Jazeera's southern Africa correspondent, said on Saturday that the MDC had little choice but to contest the run-off if they hoped to take power from Mugabe.
"The MDC has always maintained that they won't take part in a run-off ... but that will be on the agenda of this weekend meeting because if they don't take part ... they are pretty much handing victory to the Zanu-PF party," she said.
Maistry was speaking from the Johannesburg, the South African city where Tsvangirai and Tendai Biti, the MDC's general secretary, are currently staying.
Maistry said the MDC leaders were likely to discuss the possibility of forming an alliance with a breakaway faction of the MDC or with Simba Makoni, an independent candidate.
"We have also seen a slight change in the MDC stance in recent weeks," she said.
"They have been visiting different countries across the region and even going to the United Nations in New York to try and get as many world players to assist them as possible."
The MDC previously rejected the idea of a run-off and on Friday and accused the electoral commission of inflating Mugabe's number of votes by 47,000 and deflating those for Tsvangirai by 50,000.
"Morgan Tsvangirai is the president of the republic of Zimbabwe to the extent that he won the highest number of votes," Tendai Biti, the MDC's general secretary, told a news conference in South Africa.
"Morgan Tsvangirai has to be declared the president of Zimbabwe."
Under Zimbabwe election law, the run-off has to be held within 21 days of the announcement of the result.
Western powers have also cast doubt on the validity of the results.
In Washington, a US state department spokesman said that the recently announced results had "rather serious credibility problems" and doubted a run-off would be free and fair.
International rights and aid groups have also expressed alarm over political violence following last month's election.
|Mugabe has accepted the results and confirmed|
he will take part in the run-off [AFP]
There have been reports of thousands of Zimbabweans fleeing the country for fear of violence.
Human Rights Watch, the US-based rights organistaion, called the chances of a free and fair election a "tragic joke", following alleged attacks on opposition supporters by the army and other groups allied to the Zanu-PF.
But Bright Matonga, Zimbabwe's deputy minister of information, rejected the claims, telling Al Jazeera that "Zimbabwe is capable of running a very free and fair election".
He blamed the British government for attempting to undermine the elections.
"The last election was free and fair. The only violence that came up was the violence that was instituted by the British government - they paid some youths and gave them drugs."
He said it was "part of the British strategy to cause as much chaos and violence as possible ... so that they can justify the idea of forming a government of national unity".