The head of Zimbabwe's biggest opposition party has called the country's parliamentary and presidential elections "a huge farce."

"It is a sham election that does not reflect the will of the people," Morgan Tsvangirai told a press conference at his Movement for Democratic Change (MDC) party's headquarters on Thursday, the day after polls closed.

Tsvangirai, who is challenging longtime incumbent Robert Mugabe for the presidency, claimed that "administrative and legal violations" have rendered the election invalid, including intimidation and false entries on the electoral roll. 

"In our view, that election is null and void," he said, calling on regional authorities to investigate.

Zimbabwe's largest election watchdog said Thursday that the vote's credibility was "seriously compromised." The Zimbabwe Election Support Network (ZESN) received reports of polling stations systematically turning away thousands of voters in urban areas, where the MDC's support is strongest, the monitor said.

ZESN estimates that as many as 1 million people were prevented from voting.

More from GlobalPost: Zimbabwe votes in critical election (VIDEO)

Mugabe's Zanu-PF party, whose members began claiming victory as early as Thursday morning, refutes the accusations.

A spokesman told the BBC that any irregularities had affected all candidates equally, and put flaws down to a lack of resources rather than intentional fraud.

Tsvangirai is "talking absolute nonsense," Minister of Youth Development and Zanu-PF lawmaker Saviour Kasukuwere told the Guardian's correspondent, David Smith. Claiming that the ruling party had lost some key constituencies, Kasukuwere asked: "Would we rig against ourselves?"

The head of the African Union's observer mission, former Nigerian president Olusegun Obasanjo, said on Wednesday his initial impressions were of a "peaceful, orderly, free and fair vote." No Western observers were permitted to monitor Wednesday's polls.

The Zimbabwe Election Commission is due to announce its official result within five days.

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