"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 warned, calling 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 the ruling party 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 - who has ruled for 28 years since Zimbabwe gained independence from Britain in 1980 - chaired a meeting of the 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.