Zimbabwe's opposition leader said the presidential election was a "huge farce" and should be considered "null and void," alleging violations in the voting process.

Meanwhile the party of his rival, President Robert Mugabe, has declared victory.

Prime minister Morgan Tsvangirai, head of the Movement for Democratic Change (MDC), said the vote was rigged and warned that the country could plunge into a deep 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.

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

Earlier an independent poll monitoring, the Zimbabwe Election Support Network, said the poll was compromised, as up to a million people were prevented from voting.

Mugabe, who had pledged to step down in case of defeat, denied the allegations.

Some of his Zanu-PF party members went further declaring victory, although under Zimbabwe's draconian laws it is illegal to publish unofficial results.

"We've taken this election," said one senior Zanu-PF source. "We've buried the MDC. We never had any doubt that we were going to win."

The last presidential election which took place in 2008 was marred by violence. Tsvangirai won the first round but Mugabe took the final prize amid vote-rigging allegations.

The two leaders have been sharing the administration since 2009, following a deal brokered by the regional African bloc after

Final election results are expected by the beginning of next week.

Mugabe, 89, has been ruling over the former southeast African British colony since 1980.