Should Mugabe concede defeat?Send us your views
"A mooted presidential run-off ... is a sham," Tsvangirai wrote in a commentary in Monday's issue of the British newspaper The Guardian.
"Our country is on a razor's edge," he said.
He called on South Africa, Britain and the United States to step up pressure on Mugabe and his supporters to stand aside.
State media reported on Sunday that Zanu-PF had snubbed an approach from the MDC to form a unity government and was now demanding a complete recount of the presidential vote after detecting irregularities.
The MDC has denied that a unity government was ever on the table.
Mugabe chaired a meeting of Zanu-PF's politburo on Friday and was endorsed to compete in a run-off if neither of the two main contenders wins more than 50 per cent of the presidential vote.
He has ruled without interruption since Zimbabwe gained independence from Britain in 1980, but has lost support amid an economic crisis.
Zimbabwe has the world's highest rate of inflation - more than 100,000 per cent - and about 80 per cent of the work force is without employment.