The claims have been rejected by the Zimbawean government: "The Zimbabwean Electoral Commission is comprised of persons from the ruling Zanu-PF and the opposition MDC," Bright Matonga, the deputy information minister said.
"We are not surprised by these kind of reports, actually they are becoming a joke," he told the BBC.
"The organisation that is doing that, they should be ashamed of themselves because they don't see anything positive about Zimbabwe," he added.
Robert Mugabe, Zimbabwe's president, is expected to win the presidential elections against Morgan Tsvangirai, leader of the Movement for Democratic Change (MDC) and Simba Makoni, a former finance minister.
The government has also been of using the distribution of food and farming equipment to gain political advantage and denying political opponents access to the media.
For its part, the Movement for Democratic Change said in a statement on Tuesday that police had refused it permission to hold a rally in a Harare stadium.
It said it was prevented from holding its gathering because Zanu-PF had booked the stadium for four consecutive days.
|Zimbabweans are experiencing the highest |
rate of inflation in the world [AP]
"It is impossible for a political party to hold a rally at the same place for four consecutive days and the decision by the police to turn down the MDC rally on such suspicious grounds is part of the rigging process," it said.
Another report from the Observatory for the Protection of Human Rights Defenders also said there was a pattern of violence and intimidation against the government's critics.
A team from the group witnessed Zanu-PF youth members and war veterans attacking teachers at Zanu-PF headquarters in Harare, it said.
The teachers were protesting about the dire economic situation in the country, which has the highest level of inflation in the world.
"This typifies the electoral operating environment facing human rights defenders in Zimbabwe," the Observatory said.
But Simon Moyo, Zimbabwe's ambassador to South Africa, told a parliamentary committee in Cape Town on Monday that "hostile" nations such as Britain and the US should not interfere.
"There is no doubt that the forthcoming elections will be free and fair, minus external interference," Moyo said.
Zimbabwe is only admitting observers from countries it deems friendly and has not yet said whether it will accredit 300 foreign journalists who have applied to cover the elections.
Teams from the Southern African Development Community (SADC) - of which Zimbabwe is a member - have started arriving to monitor the poll.
Kingsley Mamabolo, the head of South Africa's monitors, said his group wanted to check amendments to the electoral laws passed by Mugabe.
Among the changes, police officers are now permitted to enter polling stations to "assist" illiterate people to vote.
"The perception could be that police will intimidate voters, we'll have to check that," Mamabolo said, according to the South African Press Association.
The Human Rights Watch report said that a mediation effort by Thabo Mbeki, South Africa's president, had failed to ensure that free and fair elections would be held.
The group said it had documented a litany of incidents of intimidation in Zimbabwe, mainly carried out by ruling party supporters and state security agents.