A top member of Mugabe's Zanu-PF party claimed Mugabe had trounced Morgan Tsvangirai and his Movement for Democratic Change (MDC) in Wednesday's presidential and parliamentary elections.
"We have romped [to victory] in a very emphatic manner," said the party member who asked not to be named.
"We have won all of them, including the presidential and parliamentary."
But the claim was swiftly slapped down by Tsvangirai, bidding for a third time to end 89-year-old Mugabe's 33-year rule of the troubled southern African country.
"It's a sham election that does not reflect the will of the people," he said, pointing to a litany of alleged irregularities.
"In our view this election is null and void," he added. "This election has been a huge farce."
The Zimbabwe Electoral Commission (ZEC) said the count has been completed and results are now being collated from the first vote since bloody polls in 2008, led to an uneasy power-sharing deal between Tsvangirai and Mugabe.
Tsvangirai stopped short of claiming victory himself, a move that could have enflamed tensions in a country where political violence is common.
Top MDC official Roy Bennett called for a campaign of "passive resistance”.
"I'm talking about people completely shutting the country down - don't pay any bills, don't attend work, just bring the country to a standstill."
"There needs to be resistance against this theft and the people of Zimbabwe need to speak out strongly."
Foreign diplomats and independent Zimbabwean election observers also expressed grave misgivings about the conduct of the poll.
"Up to a million voters were disenfranchised," said Solomon Zwana the chairperson of Zimbabwe Election Support Network, which has 7 000 observers.
"The election is seriously compromised."
The Catholic Church - which has 3 000 people on the ground - said it was premature to call a winner, but there was a "strong feeling" across the country that Mugabe would lose.
"If certain people feel their choice was not accepted they may resort to violence. That potential is still there," a church spokesperson said.
Since no Western groups were allowed to monitor the polls, the view of observers from the Southern African Development Community (SADC) may now be pivotal in deciding how the international community reacts.
The SADC said it will deliver its verdict on Friday.
‘Free and fair’
Meanwhile, the AU, which has been accused of whitewashing problems in the run-up to the vote, said initial reports indicated it was "peaceful, orderly, free and fair".
"Yes, there were incidences, little incidences here and there but they were not incidences that would flaw the election to the point of not reflecting the will of the people," said former Nigerian president Olusegun Obasanjo, who led the AU mission.
After years of international sanctions there had been hopes that a free election would allow Zimbabwe to reset relations with the West.
Turnout was reported to be high, with many of the 6.4 million eligible voters queuing before sunrise in the winter cold, hours before polls opened.
The lines continued well into the evening, with many marking their ballots by candle light.
Mugabe, Africa's oldest leader, is seeking a seventh term but Tsvangirai has voiced hope the election will usher in a new era.
Final results are expected within five days of the election and police had warned that anyone trying to release unofficial figures ahead of time risked being arrested.
Mugabe himself had even threatened to arrest Tsvangirai if he tried to declare an early victory.
On Tuesday the firebrand had vowed to step down if Tsvangirai was the victor, saying: "If you lose you must surrender."
Mugabe came to prominence as a hero of Africa's liberation movement, guiding Zimbabwe to independence in 1980 from Britain and white minority rule.
But his military-backed rule has been marked by controversial land reforms, a series of violent crackdowns, economic crises and suspect elections that have brought international sanctions and made him a pariah in the West.
As the economy recovers from a crisis that saw mass unemployment and galloping inflation, Mugabe loyalists insist their hero is "tried and tested" and dismiss concerns about his age.
Mugabe had focused his campaign on attacking homosexuals and on promises to widen the redistribution of wealth to poor black Zimbabweans.
Tsvangirai himself had predicted the MDC would win "quite resoundingly".
The 61-year-old former union boss won the first round of voting in 2008, but was forced out of the race after 200 of his supporters were killed and thousands more injured in suspected state-backed attacks.
Tsvangirai hoped his plans to lure back foreign investors, create a million jobs in five years and improve public services would deliver a long-awaited victory.