The bodies of four opposition activists have been found near the Zimbabwe capital, after being abducted, the Movement for Democratic Change says.
An MDC spokesman accused supporters of President Robert Mugabe of being behind the deaths, ahead of next week's poll.
The body of Harare's recently elected opposition mayor's wife has also reportedly been found, badly burnt.
Tanzania, current chair of the African Union, says there is every sign the election will not be free and fair.
Tanzanian Foreign Minister Bernard Membe said he and his counterparts from Angola and Swaziland would be appealing to their own presidents to take urgent action "so that they do something urgently so that we can save Zimbabwe".
"It is time to move on from calling it a campaign of violence. This is now terror, plain and simple"
Mr Membe was speaking at a news conference on behalf of a peace and security troika of nations from the Southern African Development Community (SADC) which is monitoring the polls.
SADC observers had witnessed people being shot dead and were now questioning the value of their presence in Zimbabwe, Mr Membe told the BBC.
His comments are the latest in a growing chorus of opinion from across Africa that the elections now appear to be fatally flawed, says the BBC's Peter Greste in Johannesburg.
A senior Western diplomat in the region has said: "The atmosphere remains violent and tense. It [the violence] is not abating and is spreading to areas to where it has not spread before, including the high density urban suburbs of Harare.
"It is time to move on from calling it a campaign of violence. This is now terror, plain and simple."
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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 ahead of the 27 June presidential run-off.
A growing number of international leaders have condemned the violence, but Mr Mugabe's allies have denied any responsibility.
The country's own independent electoral watchdog, the Zimbabwe Election Support Network, said it had at last been formally invited to monitor the poll, but only with 500 observers - a tiny fraction of the 12,000 it had hoped to deploy to keep track of the 9,000 polling stations.
Earlier, Zimbabwe's public broadcaster ZBC said it would no longer carry campaign adverts from the MDC - a decision the opposition said it would challenge.
Justice Minister Patrick Chinamasa defended the move, saying international coverage favoured the MDC and never reported the ruling Zanu-PF's position.
Correspondents say the ban on adverts will not make a great deal of difference, as news bulletins at the state-run ZBC have always favoured Mr Mugabe, only mentioning the opposition in negative terms.
There are no privately controlled radio or TV stations in Zimbabwe and only a few weekly newspapers, which most people cannot afford.
Leg irons
MDC Secretary General Tendai Biti is expected in court shortly to face treason charges.
He was arrested last week after returning from South Africa.
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The MDC says his arrest is political and is seeking his release on bail.
His court appearance on Wednesday, when he was wearing leg irons, was cut short because of a power cut in the magistrates' court.
On Thursday, US Secretary of State Condoleezza Rice is to chair an informal UN Security Council meeting on Zimbabwe, in an attempt to maintain international political pressure.
"I think that it is time for the leaders of Africa to say to President Mugabe that the people of Zimbabwe deserve a free and fair election," Ms Rice told reporters in Washington.
"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."
UN Secretary General Ban Ki-moon on Wednesday expressed concern over the level of violence.
But South Africa, Zimbabwe's neighbour, has been leading diplomatic attempts to resolve the situation and is opposed to the council having too much involvement.
South African President Thabo Mbeki met both Mr Mugabe and MDC leader Morgan Tsvangirai separately on Wednesday.
South Africa says it is not for the council to resolve disputed elections.
Many African leaders still see Mr Mugabe as a hero of the fight against colonialism and have been reluctant to criticise him in public.
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