The main opposition Movement for Democratic Change (MDC) on Sunday claimed an early lead ahead of the official announcement of results from the general election where veteran leader Robert Mugabe is fighting for an extended rule.
"This far, short of a miracle, we have won this election beyond any reasonable doubt. We have won this election," MDC Secretary General Tendai Biti told a news conference early on Sunday.
Biti, who gave partial unofficial results based on returns posted at polling stations where counting had been completed, said "in our view this trend is irreversible."
The electoral body said it would start announcing early partial returns sometime on Sunday.
Zimbabwe went to the polls on Saturday to pick a president, lawmakers and municipality councillors.
Mugabe, who has ruled the former British colony uninterrupted since independence in 1980, is up against former finance minister Simba Makoni and Morgan Tsvangirai, leader of the MDC.
On Saturday Mugabe said he would accept results even if he is defeated.
"A tsunami is breathing down our country," said Biti.
Biti warned that if the Zimbabwe Electoral Commission announced different results from those posted outside polling centres, the MDC would not accept them.
"That will clearly be dishonesty and we will not accept that.