Zimbabwe's Prime Minister Morgan Tsvangirai on Sunday warned the body overseeing this month's presidential election he will be "closely monitoring" it to ensure a fair vote.

He said vote rigging would only happen if it was allowed by officials from the Zimbabwe Electoral Commission (ZEC).

It was up to them to ensure fair play on July 31, said Tsvangirai.

"The rigging can only occur when officials at ZEC are dishonest," he told thousands of supporters gathered in a stadium in Gweru, south of the capital Harare.

"We will be closely monitoring this because we have to protect the vote, we have to protect the voter and we have to protect the outcome of the vote."

His warning came as concerns mount that the election will not be free and fair.

A scheduled early vote by the country's security forces descended into chaos as thousands of police and soldiers were unable to vote by the time the two days of polling closed last Monday evening.

Election officials blamed the disruption on problems associated with the printing of ballot papers.

"We are concerned that Zimbabwe Electoral Commission (ZEC) fails to print ballot papers so that police and soldiers vote without chaos," he said.

"All those issues undermine the integrity of ZEC.

President Robert Mugabe called early elections, hoping to prolong his 33 years in power.

The crucial vote will end the uneasy power-sharing government formed by the two leaders in 2009, after a lengthy regional mediation.

During his address, Tsvangirai accused Mugabe of plotting to rig the vote.

He also criticised Mugabe for his recent attacks on the Southern African Development Community (SADC) mediation team led by South African President Jacob Zuma.

Mugabe on Saturday accused the bloc of lying about the political situation in Zimbabwe, including the country's preparedness for the vote.

Tsvangirai won the first round of the 2008 vote but failed to get a clear majority, resulting in a run-off between himself and Mugabe. He later pulled out of the race citing violence against his supporters.

Zimbabwe has been mired in a political crisis which resulted in the collapse of the economy, following Mugabe's appropriation of white-owned farms.

Tsvangarai said his government would uphold the rule of law and promised to compensate the victims of political violence.

"There must be truth. Its only when you say the truth that you are able to get justice, and without justice you cannot heal,"he said.

He said his government would provide free education to primary school children and improve health facilities.

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