HARARE, Zimbabwe -- The main challenger to Zimbabwe's longtime president, Robert Mugabe, said Thursday the election is "null and void" due to alleged violations in the voting process, but Mugabe has denied vote rigging.

Prime Minister Morgan Tsvangirai said Wednesday's election was heavily manipulated and did not meet regional or African election standards. A poll monitoring group that is not affiliated with the state also said the poll was compromised by a campaign to stop voters from casting ballots.

Mugabe has denied allegations of vote-rigging as a smear campaign by opponents. Final results are expected by Monday.

The elections posed one of the biggest challenges to Mugabe's 33-year grip on power on this former British colony in southeast Africa.

"The shoddy manner in which it has been conducted and the consequent illegitimacy of the result will plunge this country into a serious crisis," Tsvangirai warned.

The head of African Union observer mission, former Nigerian President Olusegun Obasanjo, said late Wednesday that reports of irregularities "will be investigated but have not yet been substantiated."

Mugabe's party said Thursday it has withdrawn an unauthorized message on its Twitter feed claiming a resounding victory. The ZANU-PF party said it is awaiting the release of results by the state election commission, the only body allowed under the law to announce the outcome of massive voting on Wednesday.

Solomon Zwana, head of the Zimbabwe Election Support Network, said Thursday it has found a "wide range of problems" in the election and that the poll was compromised by a campaign to stop voters from casting ballots. The monitoring group says as many as 1 million out of more than 6 million eligible voters were not on voters' lists.

Zimbabweans voted in large numbers Wednesday in an election that was peaceful compared to disputed and violent polls in 2008. Thousands of voters lined up in Harare's populous Mbare township but by Wednesday evening all the voters had been accommodated, said polling officials. "It's a tremendous turnout," said Magodelyo Yeukai, Mbare presiding officer.

Polling officials and party agents brought blankets to polling stations so that they could sleep next to the polling boxes to make sure they were not tampered with.

Mugabe, 89, has said he would step down if he loses.

Zimbabwe's shaky government was effectively dissolved on Wednesday. Mugabe and Tsvangirai have each predicted outright victory that would avoid the formation of another coalition.

Half the population of 12.9 million was eligible to vote. The state election body has said administrative, logistical and funding problems hindered voting arrangements, but said the problems have been fixed at the more than 9,000 polling stations nationwide.

Previous elections in 2002 and 2008 were marred by allegations of vote-rigging and political violence. Rights groups say there has been little overt violence this time but noted deep concerns over voters' lists, the role of Mugabe's loyalist police and military in the voting process and bias in the dominant state media and the sole national broadcaster controlled by Mugabe loyalists.

The International Crisis Group, a research organization, said it fears a return to a protracted political crisis and possibly extensive violence if the Zimbabwe poll is inconclusive and disputed.