As a senior member of Mugabe's ZANU-PF party anonymously claimed victory in a telephone interview with Reuters news agency, Tsvangirai called a press conference in Harare declaring Wednesday's election "null and void" and "a huge farce."
Tsvangirai's main task now is to convince African Union and southern African election observers that the poll was fraudulent on the basis of a problematic voter roll that included many dead people and duplicate names.
ZANU-PF made no official victory claim over Tsvangirai and his Movement for Democratic Change party, but the comments by the party figure to Reuters may indicate the line it plans to take in the coming days.
"We've taken this election. We've buried the MDC. We never had any doubt that we were going to win," the official was quoted as saying.
Tsvangirai countered: "It's a sham election that does not reflect the will of the people."
The independent Zimbabwe Electoral Support Network, which deployed 7,000 observers around the country, more than any other observer group, said in a statement that the election was "seriously compromised by a systematic effort to disenfranchise an estimated million voters."
The group said the names of more than 750,000 voters in Tsvangirai's urban strongholds were missing from the voter roll.
"When compounded by the massive bias in the state media, the campaign of intimidation in rural areas, the lack of meaningful voter education, the rushed electoral process and the harassment of civil society, [this leaves] the credibility of these elections severely compromised," the group said.
It called on African observers to act objectively in judging the credibility of the process.
The South African Development Community Electoral Commission Forum said Thursday the election was free and fair, the process was credible, and the result should be accepted. Other groups have yet to release their assessments.
Tsvangirai has warned that his party will release its own election results Friday, if the Zimbabwe Electoral Commission fails to do so. Police have threatened to arrest anyone who announces an unofficial result before the commission weighs in.
An anonymous commenter on Facebook, who has more than 300,000 followers, claims to be a former member of Mugabe's government and goes by the name Baba Jukwa, has also promised to release the election results unofficially.
The electoral commission said voter turnout was large, but released no figure. If no presidential candidate wins more than 50% of the vote, the election will go to a runoff.
The disputed election result in 2008 led to an uneasy government of national unity. As president, Mugabe retained control over the powerful military and security sector, while Tsvangirai served as prime minister.