The security forces have been accused of backing President Mugabe
Zimbabwe's "war veterans" militia plan to intimidate voters by posing as police officers during the presidential run-off, a policeman has told the BBC.
He said they would be based inside polling stations during the vote, whose date has not yet been fixed.
It has been confirmed that police have arrested two top trade union officials.
Meanwhile, South African President Thabo Mbeki, the lead negotiator on Zimbabwe, has left Harare after talks with President Robert Mugabe.
A trade union official on Thursday said that 40,000 farm-workers and their relatives had fled their homes because of violent attacks.
The government has in turn accused the opposition Movement for Democratic Change (MDC) of staging political attacks, while saying the extent of the violence has been exaggerated.
But a South African election observer has said that the violence makes it impossible to hold a run-off.
The BBC's Orla Guerin met the police officer deep in Zimbabwe's bush, as he was afraid of being identified.
BBC undercover report from Zimbabwe
"The war veterans will be wearing police uniforms," he said.
"They will be given ranks and force numbers. They'll be part and parcel of the police deployed in every ward.
"So when people come in to vote they will see war veterans from their area in among the police, and they will be intimidated."
He said that preparations were at an advance stage - that the order to issue uniforms had already been given by provincial police headquarters.
Though opposed to the plan, he said he was powerless to stop it because if he objected he would be risking his life.
"Anything can happen," he said.
"You can be abducted, or just disappear, or your family can be endangered. You never know who is watching you. You can't trust anyone in Zimbabwe."
The opposition says its supporters are being systematically targeted
The police, he added, had been told to go out and campaign vigorously for Mr Mugabe's Zanu-PF party, and to remind people that they had won the country's freedom through the barrel of a gun.
"They are trying to threaten people into voting for them, so they do not get off the throne," he said.
"Zanu-PF are determined to continue ruling the country, and continue destroying it."
According to this officer, there are many in the junior ranks of the police who talk privately about the need for change, but dare not speak out.
He said no-one could be certain of attitudes among the senior commanders, because they had benefited greatly under the ruling party.
Many of those who fought in the 1970s war of independence went on to become police officers and soldiers and remain deeply loyal to their war-time leader, Mr Mugabe.
But many of the so-called "war veterans" are too young to have fought in the war.
Trade unionists held
Two leading Zimbabwean trade union officials have been arrested.
Lovemore Matombo and Wellington Chibebe from the Zimbabwe Congress of Trade Unions (ZCTU), which is close to the MDC, were detained after making anti-government speeches during rallies on May Day.
They are being held at Harare Central Police Station and are expected to be charged with inciting public violence.
Police say that they want them to substantiate claims that MDC supporters have been killed by ruling party supporters.
The MDC says its leader Morgan Tsvangirai won the first round of the presidential election outright and should be declared president.
According to official results, Mr Tsvangirai gained more votes than Mr Mugabe but not the 50% needed for outright victory.
The run-off is supposed to be held within 21 days of the publication of the results - last Friday - but the electoral commission head has reportedly said it could be delayed for up to a year.