Opposition leader Morgan Tsvangirai says Zimbabwe country "is effectively being run by a military junta".
He said 66 opposition supporters had been killed in political violence since March's disputed presidential elections and 200 more were unaccounted for.
The Movement for Democratic Change says its leader beat President Robert Mugabe outright. Officials say he did not get enough votes to avoid June's run-off.
Mr Tsvangirai said he would not accept a victory for Mr Mugabe in the run-off.
He said the MDC was sure to win the vote, and dispelled rumours of discussions about a possible unity government with the ruling Zanu-PF party.
Mugabe victory 'unacceptable'
"The MDC is focused on the run-off, our victory is certain," the MDC leader told a news conference in the capital Harare.
"The issue of a government of national unity before the run-off does not arise."
He was referring to comments made earlier by the defeated presidential candidate, Simba Makoni, who said supporters of Mr Tsvangirai and Mr Mugabe were in contact to try to resolve the crisis.
Mr Tsvangirai said a total of 3,000 MDC supporters had been hospitalised.
(BBC)
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