Human rights groups today accused Robert Mugabe's government of harassing and intimidating opposition supporters before Saturday's national elections.

Amnesty International cited a case on March 7, when three members of the Morgan Tsvangirai-led faction of the Movement for Democratic Change (MDC) were ordered by intelligence officers to take down election posters.

According to Amnesty, the officials forced the opposition supporters to chew the posters and swallow them.

"We continue to receive reports of intimidation, harassment and violence against perceived supporters of opposition candidates - with many in rural regions fearful that there will be retribution after the elections," said Amnesty's Zimbabwe researcher, Simeon Mawanza, who has recently returned from the country.

The US has also voiced concerns about the fairness of Saturday's election, in which Mugabe faces two serious challengers, Tsvangirai and Simba Makoni, who has broken with the president's ruling Zanu-PF party, to run as an independent.

"We call on the government of Zimbabwe, including the Zimbabwe electoral commission, to take concrete actions to address these significant shortcomings, including respecting the human rights and fundamental freedoms of the Zimbabwean people," the US state department said.

The US, which imposed sanctions on Mugabe and his inner circle after he allegedly rigged his 2002 re-election, has been barred from sending election monitors to Zimbabwe. EU observers have also not been invited, so the vote will be monitored by the African Union, as well as representatives from China, Iran and Russia.

Both the MDC and Makoni have accused Mugabe and his Zanu-PF party of trying to rig the ballot, using the security services to intimidate voters and depriving the opposition of media coverage.

"The conditions are definitely not conducive to free and fair elections. Our supporters are still being harassed, and the police are being used as weapons for intimidation," the MDC secretary-general, Tendai Biti, told Agence-France-Presse.

The opposition and human rights groups are particularly worried by the government's decision to allow police into polling stations - ostensibly to assist illiterate and infirm voters. The US state department said the move was one of several that could "preclude free and fair elections on March 29".