Some 40,000 farm-workers and their families have fled their homes in Zimbabwean election violence, a trade union official says.
"They have been accused of voting for the opposition. Most of them are either on the roadside or sheltering at some farms," said Gertrude Hambira.
Earlier, a South African observer said the country was too violent to hold a run-off in the presidential election.
There are reports that the poll could be delayed by up to a year.
A newspaper editor and a lawyer have also been arrested.
No date has been set for the second round between President Robert Mugabe and Morgan Tsvangirai, which should be 21 days after the official results.
These, announced last Friday, said that Mr Tsvangirai gained more votes than Mr Mugabe but not the 50% needed to be declared the winner.
Mr Tsvangirai, however, says the results were fixed and insists that he did pass the 50% threshold.
He has not said whether he would take part in a run-off, citing fraud and alleged state-sponsored violence against his supporters.
Ms Hambira said that people were being targeted in rural areas which voted for the opposition.
"This population represents what might be termed the swing vote between the traditional [opposition] MDC strongholds in urban areas and the Zanu-PF strongholds in the rural areas," she said.
You cannot have the next round taking place in this atmosphere; it will not be helpful
Kingsley Mamabolo South African observer
"They have been attacked by a group of militias wearing army uniforms," said Ms Hambira, General Secretary of the General Agriculture and Plantation Workers Union of Zimbabwe.
The army has denied allegations that it is involved in the violence.
Much of the political violence in recent years has been on white-owned farms, but all but 400 of these have been seized by the state and redistributed.
Of these, some 142 have been attacked since the 29 March elections, said farmers' lobby group Justice for Agriculture (Jag).
Before the seizure of white-owned farms in 2000, there were were some 4,000 white farmers in Zimbabwe, employing some 200,000 people.
Mr Mugabe blames Zimbabwe's problems on a plot for the white farmers and their western backers to reclaim their land.
Kingsley Mamabolo said South African President Thabo Mbeki, who is trying to mediate in the crisis, had sent a fact-finding mission to the country.
"You cannot have the next round taking place in this atmosphere; it will not be helpful," he said.
Those arrested are:
Reuters photographer Howard Burditt, accused of using a satellite phone illegally
Davison Maruziva, editor of the Standard newspaper, arrested for running a piece by an opposition leader
Lawyer Harrison Nkomo, accused of "insulting the head of state".
'Peace and transparency'
Earlier, the head of the Pan-African Parliament observer team, Marwick Khumalo, said Zimbabwe Electoral Commission chairman George Chiweshe had told him that the run-off could be delayed by up to a year.
"He told me it was not possible to organise an election within the 21 days required by the constitution," said Mr Khumalo, a Swaziland MP.
The opposition says its supporters are being systematically targeted
"He said the election would be organised within the shortest possible time and this would not be longer than 12 months."
Mr Tsvangirai has called for international observers to be sent to monitor the run-off - a call backed up by UN Secretary General Ban Ki-moon.
Western observers were banned from the first round by the Zimbabwe government, which accused them of bias, after they said there had been fraud in previous elections.
African Union head Jean Ping has told the BBC that the continental body would send extra observers to Zimbabwe for the run-off.
He was speaking after meeting President Mugabe, as well as the leaders of Zambia and South Africa.
"The assurances given to me were that the second round would take place in peace and transparency," he said.
If Mr Tsvangirai does not contest the run-off, Mr Mugabe would automatically win.
Mr Tsvangirai's Movement for Democratic Change says at least 25 of its supporters have been killed since the relatively peaceful first round on 29 March.
But police and officials from Mr Mugabe's Zanu-PF party have accused the MDC of staging attacks, while accusing the MDC of exaggerating the scale of the violence.
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