Zimbabwe Prime Minister Morgan Tsvangirai has dismissed yesterday's election as a "huge farce".
He said the results were invalid because of intimidation and ballot-rigging by President Robert Mugabe's ruling party, which is claiming victory.
President Mugabe's ZANU-PF party has claimed a landslide victory in the elections that would secure another five years in power for Africa's oldest head of state.
"This has been a huge farce," Mr Tsvangirai told a news conference at his Movement for Democratic Change party's headquarters in Harare.
"In our view, that election is null and void," Mr Tsvangirai said.
Releasing unofficial results early in Zimbabwe is illegal, and police have said they will arrest anybody who makes premature claims about the result.
Election authorities were due to announce results by 5 August.
But a senior source in Mr Mugabe's ZANU-PF party, who asked not to be named, said the outcome was already clear.
"We've taken this election. We've buried the MDC. We never had any doubt that we were going to win," the source told Reuters by phone.
Riot police took up positions outside the party's headquarters in central Harare and other key locations in the capital. MDC offices appeared to be almost deserted.
An independent election monitor in Zimbabwe, who also could not be named for fear of arrest, said early results were looking like a "disaster" for Mr Tsvangirai, who was making his third bid to unseat 89-year-old Mr Mugabe.
Responding to the ZANU-PF claim, a high-ranking source in Mr Tsvangirai's MDC party described the election as "a monumental fraud".
The MDC will hold an emergency meeting later today.
Several political sources told Reuters key MDC members had lost their seats, even in the capital, Mr Tsvangirai's main support base since he burst onto the political scene 15 years ago.
The head of an African Union observer mission said last night the polls appeared at first glance to be "peaceful, orderly and free and fair" - an assessment at odds with the view of the MDC and of independent entities monitoring the poll.
Earlier today, the Zimbabwe Election Support Network (ZESN), the leading domestic monitoring agency, said the credibility of the poll was seriously compromised by large numbers of voters being turned away from polling stations in MDC strongholds.
"It is not sufficient for elections to be peaceful for elections to be credible," ZESN chairman Solomon Zwana told a news conference.
"They must offer all citizens ... an equal opportunity to vote."
The United States, which has sanctions in place against Mr Mugabe, had also expressed concern about the credibility of the vote, citing persistent pro-ZANU-PF bias in the state media and partisan security forces.
Other worries centred on the voters' roll, which was meant by law to be released in electronic form to all parties before the poll, but which has still not been made available.