Zimbabwean presidential hopeful Morgan Tsvangirai described the election as a "huge farce" just hours after Robert Mugabe's allies claimed victory, and warned the country faced a serious crisis.
"It's a sham election that does not reflect the will of the people. In our view this election is null and void," Tsvangirai said, pointing to a litany of alleged irregularities in Wednesday's vote.
"This election has been a huge farce," he said.
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"The shoddy manner in which it has been conducted and the consequent illegitimacy of the result will plunge this country into a serious crisis."
Mugabe's allies claimed an "emphatic" victory, but local observers have called the vote "seriously compromised".
Election day had passed off without widespread violence, but critics pointed at a flawed electoral roll, among other problems.
Mugabe's critics have accused him of rigging the poll to extend his 33-year-rule.
Unofficial results compiled by civil society groups appear to show that he did surprisingly well in urban areas, where he normally falls flat.
Election organisers are expected to publish official results within five days of the vote.
