Zimbabwe's Commercial Farmers' Union (CFU) said the farmers were abducted, assaulted and thrown off a moving vehicle in Chegutu district, 100 km (62 miles) west of Harare.

"Serious injuries have been inflicted on all three, all of whom have sustained serious head injuries," it said in a statement.

The run-off vote was won by President Robert Mugabe, the only candidate.

The opposition Movement for Democratic Change (MDC) and human rights groups have accused Mugabe of using the army, militia and war veterans to intimidate Zimbabweans into voting for him. The government denies the accusations.

MDC leader Morgan Tsvangirai, who defeated Mugabe in the first round in March, withdrew from the second ballot, saying nearly 90 opposition supporters had been killed in government-backed violence.

CFU President Trevor Gifford refused to comment on whether the attacks on the farmers were politically motivated.

"You'd have to talk to the people involved," Gifford said.

War veterans and ruling ZANU-PF militia invaded some white-owned farms immediately after the first round vote on March 29, saying they were harbouring returning farmers who sought to reclaim their old properties.

The attacks since Sunday were aimed against a group of farmers who successfully challenged Mugabe's attempt to acquire their properties at a regional tribunal.

The three injured farmers were named as Michael and Angela Campbell, who led the legal challenge, and Ben Freeth who was also associated with it.

"Mrs Campbell sustained two broken arms as well as serious head injuries. Mr Campbell also has serious head injuries as well as a broken collar bone, while Mr Freeth sustained serious head injuries and it is feared he may lose an eye," the CFU statement said.

A blog by the Sokwanele Civic Action Support Group, an activist organisation, showed photographs of the Campbells and Freeth in hospital with severely bruised faces.

The CFU accused the police of failing to take action against against the attackers.

Police spokesman Wayne Bvudzijena said 16 people had been arrested for robberies in the district, while a search was in progress for more members of the group.

"We have arrested 16 people who were trying to move into farms and going about stealing and robbing people of their property," Bvudzijena said.

"These are cases of common criminality, with a group of about 40 going around victimising people. We have now stationed armed police officers on the farms."