"Since no candidate has got the majority of the valid vote, a second election shall be held on a date to be advised by the commission," Sekeramayi said.
Under Zimbabwe election law, the run-off has to be held within 21 days of the announcement of the result.Mugabe to stand
After the results, an aide to Mugabe 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.
However, he questioned whether the results accurately reflected support for the incumbent.
"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.
Simba Makoni, a former finance minister, came third in the election with just 8.3 per cent. He is widely expected to back Tsvangirai in any run-off.
The MDC has previously rejected the idea of a run-off and on Friday accused the commission of inflating Mugabe's number of votes by 47,000 and deflating those for Tsvangirai by 50,000.Tsvangirai 'president'
"Morgan Tsvangirai is the president of the republic of Zimbabwe to the extent that he won the highest number of votes," Tendai Biti, the party's general secretary, told a press conference in South Africa.
"Morgan Tsvangirai has to be declared the president of Zimbabwe."
Biti said that a run-off could not be held "for the simple and good reasons that that country is burning" amid violence and an economic crisis.