Tendai Biti, MDC secretary general told AFP, "The conditions are definitely not conducive for free and fair elections. Our supporters are still being harassed and the police are being used as weapons for intimidation."
Biti said Mugabe, who has ruled since Zimbabwe's independence from Britain in 1980, was reneging on agreements for the framework for the elections reached during talks mediated by Thabo Mneki, South African president.
During those negotiations, "the MDC's position was that the police had been abused and used systematically to generate intimidation and threats and we agreed that they should not be allowed in polling stations," Biti said.
"But Mugabe has brought back the old order by allowing the police back in polling station. It is our view that Mugabe who is a participant in the game cannot change the rules when the game is being played."
The government has caused alarm in opposition ranks by allowing police into polling stations to assist any voter who is either illiterate or infirm.
That decision to allow police inside polling booths was among the issues of concern highlighted in a statement by the State Department which warned could "preclude free and fair elections on March 29".
The Zimbabwe police dismissed such accusations as part of a Western ploy to discredit the elections.
Wayne Bvudzijena, chief police spokesman told AFP, "We get these statements each time we have elections and the idea is to declare that the elections were not held in a free and fair atmosphere if they don't like the results."
"Everyone is campaigning freely. We have only had scattered incidents of violence," he said.