Mr Mugabe has defied calls for Friday's vote to be called off
South Africa's governing ANC party has accused the Zimbabwean government of "riding roughshod" over democracy and said a fair election is not possible.
A BBC correspondent says this is the strongest statement so far by the ANC on Zimbabwe and a sign of mounting diplomatic pressure on its government.
The regional bloc Sadc is due to hold an emergency summit on Zimbabwe's political crisis, reports said.
President Robert Mugabe insists the vote will go ahead despite UN concerns.
Speaking at a rally in the town of Banket, he said: "They can shout as loud as they like from Washington or from London or from any other quarter. Our people, our people, only our people will decide and nobody else."
Zimbabwe's opposition Movement for Democratic Change said it had confirmed in writing on Tuesday its withdrawal from the presidential race.
The pull-out was announced at the weekend by the party's leader, Morgan Tsvangirai, who cited widespread violence as the reason.
'Compelling evidence'
Southern African leaders will meet for emergency talks called by the Southern Africa Development Community (Sadc) in Swaziland's capital, Mbanane, the Tanzanian president's office said in a statement.
Morgan Tsvangirai, has been completely outmanoeuvred... The outside world, which mostly sympathises with him, can do nothing whatever to help him
The announcement came hours after the ANC said it was "deeply dismayed by the actions of the Zimbabwean government - which is riding roughshod over hard-won democratic rights".
It said it could not remain "indifferent to the flagrant violation of every principle of democratic governance".
And the party referred to "compelling evidence of violence, intimidation and outright terror". The comments come a day after the UN Security Council unanimously agreed to condemn the violence in Zimbabwe and said a fair election would be "impossible".
The BBC's Peter Biles in Johannesburg says that coming from South Africa - the most powerful country in the region - the ANC statement is a further sign of President Mugabe's growing isolation.
Mr Mbeki has been adopting a policy of "quiet diplomacy" towards Zimbabwe.
There had been reports on Tuesday that Mr Mbeki would return to Zimbabwe to hold talks with government and opposition leaders. However, his office denied that that was the case.
In separate comments, ANC leader Jacob Zuma, who rivals Mr Mbeki as South Africa's most powerful man, said the situation in Zimbabwe was "out of control" and called for urgent intervention by the UN and regional Sadc grouping.
Meanwhile, Mr Tsvangirai said he would leave the Dutch embassy in Harare sometime over the next 48 hours.
Mr Tsvangirai, who took refuge there on Sunday night, hours after pulling out of this Friday's vote, said the Dutch ambassador had received assurances from the Zimbabwean authorities about his safety.
Senegalese President Abdoulaye Wade, in a statement about his attempts to mediate in Zimbabwe, said Mr Tsvangirai had been fleeing soldiers when he took refuge.
'Cry baby'
BBC world affairs editor John Simpson, who is in Harare, says few people in Zimbabwe know Mr Tsvangirai has pulled out of the race as official media barely ever mention him.
Meanwhile, Zimbabwe's UN ambassador Boniface Chidyausiku told the BBC Mr Tsvangirai, who was briefly detained five times while campaigning, had never had his movements restricted by the authorities.
"He's a cry baby... He has been free to move wherever he wanted to move," he said.
ZIMBABWE AND ITS NEIGHBOURS
Zimbabwe's opposition wants neighbouring countries to persuade
Robert Mugabe to step down. So how are relations changing?
South Africa's leader Thabo Mbeki remains the key mediator. He has not
criticised Mr Mugabe, despite pressure from the ruling ANC.
Zambian President Levy Mwanawasa called Zimbabwe a "regional
embarrassment", before suffering a stroke on 29 June.
Angola's President Jose Eduardo dos Santos is one of Robert Mugabe's closest allies. He has urged Mr Mugabe to end the violence.
Botswana said Zimbabwe's 27 June run-off vote was so flawed by violence
that it could not be considered legitimate.
Namibia is an ally of Robert Mugabe. It wants to re-distribute white-owned farms to black villagers. It has not criticised the violence.
Mozambique has hosted some white farmers forced out of Zimbabwe when their land was seized. It is seen as sympathetic to the opposition.
Tanzania's ruling party has a history of backing Robert Mugabe's
Zanu-PF party. Its foreign minister has condemned the violence.
DR Congo's President Joseph Kabila is an ally of Robert Mugabe who
sent troops to help his father, Laurent Kabila, fight rebels.
Malawi is seen as neutral. But 3m people from Malawi are in Zimbabwe
and many were badly hit by the farm invasions.
Robert Mugabe, quoted by the pro-government Herald newspaper, accused Western countries of "telling a lot of lies about Zimbabwe" in order to justify an intervention
Former UN High Representative in Bosnia and Herzegovina Paddy Ashdown, a British politician, warned Zimbabwe's violence could descend into genocide like that in Rwanda in 1994
The International Cricket Council said it would consider whether to ban Zimbabwe from international cricket at a meeting in Dubai next week
An African election observer, who does not want to be named, told the BBC torture was "the order of the day" in Zimbabwe
The opposition says some 86 supporters have been killed and 200,000 forced from their homes by Zanu-PF militias but the ruling party blames the MDC for the violence.
Meanwhile, the BBC's Themba Nkosi in Bulawayo, south west of Harare, is reporting that youths and villagers from an MDC faction have this week been ambushing and attacking pro-Zanu-PF so-called war veterans in the area.
In Mashonaland West, another BBC contributor said Zanu-PF youths abducted MDC councillor-elect Lavender Chiwaya at his house on Tuesday morning, beating him before he was released by another MDC official.
The MDC won the parliamentary vote in March, and claims to have won the first round of the presidential contest outright.
According to official results, Mr Tsvangirai was ahead of Mr Mugabe but failed to gain enough votes to avoid a run-off.
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