African states monitoring Zimbabwe's campaign say mounting violence could make a free and fair vote impossible.
Tanzanian Foreign Minister Bernard Membe, who heads a troika of observer states, told the BBC that monitors had witnessed people being shot dead.
As arrests of opposition activists continued, they were questioning the value of their mission, Mr Membe said.
In the latest violence, four abducted activists have been found dead, the opposition reports.
Their bodies were found near the capital Harare, according to the Movement for Democratic Change (MDC), led by presidential contender Morgan Tsvangirai.
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The body of Harare's recently elected opposition mayor's wife has also reportedly been found, badly burnt.
The UN Security Council is to hold an informal meeting on Zimbabwe on Thursday, in an attempt to maintain international political pressure.
Opposition 'derailed'
Mr Membe was speaking at a news conference on behalf of the three nations from the Southern African Development Community (SADC) monitoring the polls.
He said that he and his counterparts from Angola and Swaziland would be appealing to their own presidents to take urgent action "so that we can save Zimbabwe".
"The first impression we have is that if the elections were to take place today, these elections would never be free and fair... because... the report we received still indicates that violence is escalating throughout Zimbabwe," he told the BBC.
"We have received a report that says on the 16th of June this year, as the observers were being deployed to those various stations, two people were shot dead.
"Of course, it scared most of these observers to the extent that they had to pose the question of why are we here then, and what are we doing?"
"There is a derailment of Mr Tsvangirai wherever he wants to go to campaign, he's detained at police stations," Mr Membe added.
His remarks are the latest in a growing chorus of opinion across Africa that Zimbabwe's elections now appear to be fatally flawed, the BBC's Peter Greste reports from neighbouring South Africa.
On Wednesday, the head of another African observer mission in Zimbabwe - Marwick Khumalo of the pan-African parliamentary group - warned he would not endorse the run-off vote if current levels of violence continued.
US Secretary of State Condoleezza Rice is due to chair the UN Security Council meeting.
"You cannot intimidate opponents, you cannot put opponents in jail, you cannot threaten them with charges of treason, and be respected in the international community," she told reporters in Washington.
Observer numbers slashed
Zimbabwe's own independent electoral watchdog, the election support network, says it has at last been formally invited to monitor the poll but only with 500 observers.
That is a tiny fraction of the 12,000 the network had hoped to deploy to keep track of the 9,000 polling stations that will be opened on election day.
The network has been credited with helping to keep the first round of the election relatively free and fair but even now its members have come under attack, our correspondent says.
One of its observers was murdered earlier this week.
The MDC says at least 70 of its supporters have now been killed and 25,000 forced from their homes in a state-sponsored campaign of violence.
Its secretary general, Tendai Biti, is expected in court shortly to face treason charges. He was arrested last week after returning from South Africa.
Zimbabwean public broadcaster ZBC has also announced it will no longer carry campaign adverts from the MDC.
There are no privately controlled radio or TV stations in Zimbabwe and only a few weekly newspapers, which most people cannot afford.
(BBC)
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